What causes the fog on car windshields on a cold day

It is an all too common occurrence for people living in cold climates. You wake up in the morning, get into your car parked in the chilly weather overnight and before you can turn on the engine, the windshields start fogging up forcing you to turn on the defrosters / air-conditioning and just sit and wait as the fog clears. But what causes the fogging? How come it usually happens only after you enter the vehicle?

Humidity in cold vs hot air:
The key to understand fogging in a car is the fact that hot air can hold more water (humidity) than cold air. In fact, hot air can hold up to twice the amount of moisture than cold air. You might have experienced this in the form of dry skin or chapped lips in the cold weather which is a direct result of the cold air being less humid and so readily taking away your body moisture.



Another key phenomena is that when hot humid air comes in contact with a colder surface, the water vapor condenses forming a fog. This is also easily observable when we exhale on a cold day. The air coming out of our body is warm and humid and as soon as it comes in contact with the cold air outside, the vapor condenses forming a visible fog.




What happens when I enter my car:
When the car is parked for an extended period in cold weather, the humidity levels on the outside and inside reach an equilibrium. However, as soon as the driver enters and starts breathing inside the car, the air inside the car gets warmer and humid because of that person's warm breath and any other moisture they might have carried in (like water droplets on a jacket, vapor from a hot coffee etc.). This relatively hot and humid air starts condensing and fogging up as soon as it comes in contact with the cold windshields. 

Tips:


While it is unavoidable, a few tips to deal with foggy windshields - 
  • If you can avoid it, don't carry any hot beverages into the car as you enter.
  • Carry an absorbent piece of cloth to wipe away the fog on the windshield. Not only is this more time efficient than running defrosters but is also environmentally friendlier.
  • Run the air-conditioning on cold setting first and gradually rise the temperature. As discussed above, cooling down the air in the car reduces the chances of your humidity being carried by air and condensing on the wind shield. By the time you turn it on to warm setting, the air conditioning system would have already cleared the humidity.

What is the farthest point from the center of the Earth

Mt. Everest is the highest peak on the planet and so it is reasonable to assume that the peak of Mt. Everest is the farthest point from the center of our planet. However, the peak of Chimborazo volcano in Ecuador is the actual farthest point. This is true despite Mt. Everest being 29,029 ft above sea level and Chimborazo being just 20,549 ft above sea level. In fact, Chimborazo is not even the highest mountain in the Andes mountain range that it is a part of.


Elevation of Mt. Everest and Mt. Chimborazo relative to sea level
Why is a shorter peak farther away?
Chimborazo's location along the equator of the Earth is the reason why it takes the honor of being the farther point from the center. 
Like most celestial bodies, Earth is rapidly spinning on its own axis and this rotation causes a bulge along the equator, commonly known as equatorial bulge. This bulge makes the Earth 42.77 kilometers wider along the equator than it is along the poles. This means that any point near the equator is going to be farther away from the center of the planet than any point near the poles. 
The image above shows the approximate positions of these two peaks. Chimborazo is ~1° south of the equator whereas Everest is ~28° north of the equator. This proximity to the equator results in Chimborazo's peak being much farther away from the center of the Earth. 

Can women tolerate more pain than men?

That women have a higher tolerance for pain than men is a widely accepted assertion and it sounds compelling because of the reasoning that women evolved better pain tolerance because they needed to give birth. Pushing a human through birth canal can indeed be painful! However, I was always skeptical of this assertion partly based on anecdotal evidence and partly based on the other side of the argument that men should have evolved better pain tolerance because they were far more likely to get injured in the hunter gatherer societies.

What do researchers say?
A couple of definitions before looking at related research:
Pain Threshold: The point at which a person starts perceiving pain. For example, just placing a needle on the skin doesn't cause pain but as you keep pushing it against the skin, there is a point at which one starts perceiving pain which would then be their pain threshold. This is completely physiological.
Pain Tolerance: The point at which a person can no longer tolerate pain. In other words, the maximum pain a person can tolerate without breaking down. This is partly physiological but is also considerably influenced by external factors like culture, gender, situation etc.


Research spanning several decades and hundreds of subjects across different cultures has pretty much concluded that men have both a higher pain tolerance and pain threshold. What's more, when women were asked to focus on the emotional aspect of the pain, their threshold and tolerance dropped further while it had no affect on men[1].

Aren't men faking it?
Pain related studies are usually self reported, as in researchers rely on their subjects to tell them when and how they feel pain. This leads to the obvious skepticism that men might be faking it because of the societal factors which usually train and expect men to be tougher and capable of withstanding more pain.

This criticism is not totally unfounded. In fact, studies found that when compared with each other, Indian men reported higher threshold and tolerance than their American counter parts and both sexes reported higher pain tolerance when the researcher working with them is the opposite gender[2]. This strongly suggests that societal factors have a significant influence on the pain tolerance / threshold exhibited.

However, that doesn't mean that there is no physiological aspect to it. There were other studies that focused on measuring pain objectively by relying on physiological responses. In one such study, they measured how much a person's pupils dilated when subjected to pain (pupils dilate under pain and humans cannot voluntarily control their pupils). This study too found that men have a higher threshold and tolerance for pain[3].

Conclusion:
Based on the studies referred to above, it is safe to conclude that men can handle pain better. Having said that, it is also true that the difference isn't as much as men want it to be because they are partially faking it.

References:

Why do black people exist at all?

Who knew that a simple trait like skin complexion could cause so much trouble for our species? While we are slowly but definitely moving towards a post-racial society, the symptoms of the systematic oppression and abuse of black people can still be felt. Across cultures lighter skinned people have generally enjoyed higher status and better opportunities compared to their darker counter parts. This inequality wouldn't have existed if black people didn't exist in the first place. So why do they even exist?


Before trying to find out why, let us establish that it is a valid question. Take a good look at this chimp and imagine what he would look like if we were to shave his body hair. Pay attention to his mouth and chest areas. That is right - chimps and most other primates when shaved would look more like Brad Pitt than Will Smith. They are all whites! If all of our evolutionary cousins are whites, why do black humans exist?


It all started millions of years ago when humans started losing body hair. You can read theories here on why only humans lost their body hair but not thousands of other mammals. 

Harmful effects of UV radiation:
While there are several benefits to losing body hair, one obvious drawback is increased exposure to harmful Ultra Violet radiation from the Sun, especially while walking naked in the hot African Savannah like our early ancestors did. UV radiation can destroy a molecule called Folate in our body, the deficiency of which can cause birth defects. It can also significantly increase the likelihood of developing skin cancer. It has been found that not only does skin cancer kill people but it kills them at a very young age severely cutting into a person's reproductive years (a study found that Albino Africans who develop skin cancer usually die in their 30s. If skin cancer can kill people in their 30s today, it is reasonable to assume that it killed our ancestors at a much younger age).

Dark skin protects against UV radiation:
Turns out that dark skin protects both from skin cancer and Folate damage (how exactly dark skin fights UV light is beyond the scope of this article). In fact, researchers found that African Americans are 1/50th as likely as White Americans to be diagnosed with skin cancer. Since dark pigmentation offered better protection against skin cancers and Folate damage, people with darker skin lived longer and had a better chance at reproduction and were able to pass on their genes which gradually led the population to grow darker. On the other hand people with pale skin died before getting a chance to reproduce and thus gradually got weeded out of the population.

Why do white people exist then?
About 120K to 60K years ago, dark skinned humans started migrating out of the tropics towards poles with low UV radiation. When this happened dark skin's protection against UV light no longer offered an evolutionary advantage. Instead, pale skin became more advantageous because it was better at absorbing sunlight which the body needs to make Vitamin-D. (the reason why darker people in colder climates are often prescribed Vitamin-D supplements).

Summary:
To summarize, humans  evolved dark skin because it offered protection against UV light and thus is prevalent in hotter climates like Africa. Pale skin bounced back when humans migrated out of Africa because it protects against Vitamin-D deficiency in colder climates like Europe.

Is coercive sex natural?

Short answer, yes!

Coercive sex (also called rape) is considered a first degree crime in most cultures and perpetrators face severe punishments, sometimes even a death sentence. While other crimes like premeditated murder, torture, child molestation etc. can be seen as exclusive to humans, coercive sex isn't.

Rape in animal kingdom:
Most of the animal kingdom exhibits courtship behavior where typically the male tries to get a female's attention either by demonstrating his strength, fighting off other males or sometimes even bribing her with food. This is pretty much how it works in humans where the male asks a woman on a date, presents what he has to offer (in a subtle way for the most part) in an attempt to impress her and mate with her. 

However, there is a darker side in the animal kingdom that doesn't exhibit any courtship. The males engage in copulation using coercive methods and to make things worse, sometimes a group of males 'corner' a female and engage in coercive sex. This behavior has been observed in several species like ducks, dolphins and even our close relatives, chimpanzees. 

Contrary to the romantic courtships, these species even have a sexual arms race going on between their males and females. Reproduction is an expensive process for females in any species and so the females in species that are subjected to rape have started evolving several mechanisms to prevent forced fertilization. For example, it has been observed that female ducks have false passages in their vagina and when being coerced by a male that they don't prefer to mate with, they can direct his penis into the false passage and thus avoid fertilization. Like wise, with a partner of their choice, they would route his penis to the fertile zones in the vagina. So, not exactly avoiding forced sex but avoiding unwanted fertilization. 



Should we give the sexual offenders a break?
This however doesn't mean that rape is just an extension of male sexual desire and is thus justified. Just because something happens in the nature doesn't make it right. Nature, driven by evolutionary selection is plagued with disease, murder, rape, suffering and inefficiency and thus Darwinian mechanisms should definitely not serve as the basis when writing our rule book. Thanks to the wonderful brain gifted to humans by evolution, we are now capable of looking beyond our evolutionary instincts and determine that consensual sex is in the best interest of our species.

It however raises a question about the severity of punishment. If there is a biological wiring that prods some men towards committing sexual assault, should it be punished by death? What do you think?

Why is incest not cool?


Avoiding incest is a learned behavior:
Sure incest (in-breeding) is a fantasy for some but most of us loathe the idea of mating with family members. This is obviously a learned behavior and there is nothing inherently disgusting about having sex with your family members. That it is a learned behavior is obvious because there are certain human cultures where incest relationships are quite normal. We are not even talking about some remote tribes. For example, in most parts in India it is perfectly fine to marry ones cousins. In several isolated cultures like islands, incest is inevitable.

However, in general most cultures do not allow such relationships especially between closely related members, say brother/sister or dad/daughter. One would obviously wonder why cultures evolved this way. After all, bonding between family members is usually much stronger and unconditional compared to a stranger you meet in high school. So marriages among family members will probably be more successful which is good for the society.

How does in-breeding hurt?
Turns out, the reason could be the fact that babies born to related parents are more likely to develop genetic disorders. So one can hypothesize that people over several generations have observed these genetic disorders in babies and gradually veered away from inter-family mating. 

Now the science side of the story. Why is a baby born to a dad and his daughter more likely to develop genetic disorders? To answer this question we need to first understand how we inherit genes from our parents.

Genetic inheritance:
Organisms usually have two copies of genes for every characteristic (lets use the gene that determines obesity potential as an example) obtained one from father and one from mother. These genes can be of two types - dominant allele (usually the good trait) and recessive allele (the bad trait). The bad trait is expressed only if both copies are of the recessive type.
Lets consider the following combinations of dominant (G) and recessive (g) alleles for obesity potential in an individual - 
As you can see, organisms can carry these 'bad' genes without expressing the traits and thus evolution never gets a chance to weed them out completely from the gene pool.

Now imagine a pair of unrelated individuals that met in high school. Both of them will be carrying certain recessive alleles but it is less likely that they will be carrying the recessive alleles for the same trait because they are unrelated. For example, the guy might be carrying a recessive allele for obesity while the girl might be carrying a recessive allele for diabetes without expressing these traits because they also have the dominant allele in them. However, if you consider a dad and a daughter, there is a very high chance that both of them carry the recessive allele for the same trait, lets say obesity, because they are related. Now, their offspring is very likely to inherit the recessive allele from both parents and end up with a 'gg' configuration and thus obese. 

This illustration should help. The high school buddies carry recessive alleles but for different traits. So, while it possible that they pass on these recessive alleles to their baby, it is impossible for the baby to end up with two recessive alleles for the same trait. However, in the second case, the dad and daughter carry recessive allele for obesity and thus their baby has a good chance of being obese.

Obesity is obviously just an example but you can see how in-breeding can result in various genetic disorders by increasing the likelihood of inheriting two recessive alleles.

So the take away from this article? Well, don't sleep with your daughter.

Why does a flame burn upwards?

We are familiar with how flames burn. Let it be a tear shaped candle flame or a forest fire, it always burns upwards. We will look into the details later, but the following image released by NASA says it all - 
An illustration of how candle burns on earth and in zero gravity
Courtesy: http://science.nasa.gov/
Gravity does the magic: 
Its obvious from the above illustration that gravity has a role to play. However, one can't help but wonder why a flame would go upwards where there is strong gravitation pull like on earth and remain spherical where there is no gravity like on a space station (Is there really no gravitational pull in space?). The answer is buoyancy.

When you start a fire, fuel combines with oxygen to release heat, light, carbon dioxide, water vapor, soot etc. The heat energy generated in this process heats up the air around the flame which in turn reduces its density** (a good analogy is water vapor which is hotter and less denser as against water which is colder and heavier).

This hot air around the flame then starts rising up and the surrounding colder heavier air rushes down into its place accelerating the hot air upwards which in turn causes the flame to shoot up (a good analogy here is a wooden log dipped in water. the buoyancy pushes the log to the surface). Basically the buoyancy shoots the flame up. It is this hot air rushing upwards that also causes fire to flicker.

The following image shows what happens to the air around a flame.
Hot air around the flame rises up shoots the flame upwards
However, in a micro gravity environment, there is no reason for the heavier air to fall down and rush into the place of the lighter air. So the flame just remains a spherical blob as you would imagine. 

So there you have it. A combination and gravity and gravity induced buoyancy causes the hot air to rise up causing the flames to point upwards here on earth.

**Just in case you are curious as to why hot air is less denser than cold air in the first place - the heat gives the molecules in the gas higher (kinetic) energy. So these molecules can move faster and further apart because they now have the energy needed to overcome their binding forces. The molecules being farther apart means that the material is less dense.

The second-hand illusion - Does the clock stop when you do not look at it?



How to see the illusion: 
Rotate your head as much as you can away from the screen and then quickly shift your gaze back onto the clock above. The more you rotate your head and the faster you shift you gaze back onto the clock, the easier it is to catch the clock in action. The clock would appear as if it is still for a moment. In other words, the exact second at which you look at the clock appears to last longer than a second. Try this a few times if you can't see the illusion right away. The illusion is easier to see with bigger physical clocks in case you have one around you.

Many people report observing this behavior even though they don't realize that it is an illusion and that it is a normal experience with a scientific explanation.

Why does this happen?
Now to the why part. As mentioned above, the key to observing the second-hand illusion is a quick shift in gaze. Although quick shift in visual attention is a routine one doesn't pay much attention to, your brain has to resort to some sophisticated gap-filling to make the process appear seamless. Basically, when you move your eyes rapidly, you cut off visual input to your brain for a brief period and instead of showing darkness for that duration, the brain provides a seamless visual experience by filling in the duration of rapid eye movement with whatever it sees after the eyes settle.

In the case of the second-hand illusion, the first thing you see after a quick shift in gaze is the clock. So, the brain fills in the time you have taken to shift your gaze (lets say a quarter second) with whatever time stamp it sees first after the eyes settle. So, the second you see immediately after shifting your attention back to the clock would appear to last for one and a quarter second giving the illusion of a still clock.

This visual input filling done by the brain is not usually noticeable and is pretty useful because it provides a continuous story of the world around you. It is only when you look at things that are designed to move with precise regularity (like a clock made by humans) that you are able to observe this.

Note that this illusion can happen with other senses as well, not just visual. Although much difficult to observe, the same illusion happens when you shift your hearing attention from one source to the other (ever observed the first ring when calling someone on phone last longer than the following rings?). Similarly with other senses as well.

Were you able to observe the second-hand illusion now or did you ever observe it before? Let us know in the comments.

How come wheels rotate backwards in movies?

Most of us must have observed in ads / movies how the wheels of a car rotate backwards or even stay still even though the car itself is moving at a breakneck speed. See the following video to see this illusion in action. Notice how the wheels appear to be spinning backwards, forwards and at a certain point even appear to be not rotating at all.

This illusion is called Wagon-Wheel effect (no clue why it is called so). So what causes this illusion?

Camera basics:
To understand why we see the wheels spinning backwards, we need to understand how video recording works. Videos are basically a series of photos played so fast that our naked eye perceives it as continuous motion. For example, most movie cameras shoot at 24 frames per seconds (fps) which means they take 24 photos per second. When we go to a movie, these photos are played back at 24 fps and perceived as continuous motion by our eyes (when in panic human eyes are known to operate at a much higher frame rate but in relaxed state they operate at about 20 fps).

Explanation:
With the understanding that videos are actually discreet shots in time, it is easy to see why wagon wheel effect occurs. Imagine a camera that operates at 1 fps and a wheel that is spinning 340° clockwise per second. The first row in the following illustration shows what each of the photos shot by the camera would look like. Now imagine these photos being played one after the other to a human eye that operates at 1 fps. It sees counter clockwise motion!

Illustration showing various stages in wagon wheel effect

The second row in the above illustration shows why the wheel would appear still in some cases. This happens when the time between individual shots by the camera exactly matches the time taken by the wheel to complete n-rotations. 

Artificial Illumination:
While camera recording is one way to observe wagon wheel effect, artificial illumination also results in similar effects. Lighting powered by alternating current actually flickers about 50 times per second (in other words, the bulb turns on and off 50 times per second). As an object is rotating, depending on the frequency of the flicker, certain positions of the wheel will not be observed by the observer thus creating the illusion.

Continuous Illumination:
It is common knowledge that wagon wheel effect can also be observed by naked human eye under continuous illuminating conditions like sun light. A simpler explanation for this is that human eye, like a camera, actually takes snapshots of the world it sees and thus is no different from a movie camera observing wagon wheel effect. While this is definitely true, many scientists agree that this only part of the answer. There is another somewhat more complicated reason as to why human eye can directly perceive this effect. Unfortunately that is a little too complicated for this article. Curious readers can visit a library.




Why is it that humans cannot breathe under water?

If only humans were able to breathe under water, we could have ruined the oceans as well. Too bad 70% of the planet is water and not so habitable. (To give credit where it is due, humans still managed to screw the oceans to a large extent).

So, why can't we breathe under water? The obvious answer is that our lungs are not made for absorbing oxygen from water! So a better way to phrase the question would be - 
How come humans did not evolve with lungs that can breathe on land and water?

Before we answer that, lets refresh some basics and clear some myths - 
Its common knowledge that water is basically a combination of Hydrogen (2 parts) and Oxygen (1 part). This unfortunately leads to the misconception that fish somehow breathe this oxygen that forms the water molecules. However fish are not able to break the bonds in water molecules to meet their Oxygen needs. Instead they breathe Oxygen that is dissolved in water. 

Many liquids including water can hold some dissolved oxygen (DO). It is either directly absorbed from atmosphere or is produced by the aquatic plants / algae. The DO is often used as an indicator of a water body's ability to support aquatic life. It is the same oxygen that we breathe from air but dissolved in water (Two molecules of Oxygen double bonded).

One key difference though is its availability. The amount of oxygen available in water is much lesser than what is available in air. For a given volume, air has approximately twenty times more oxygen than water.

A graphic to demonstrate the levels of Oxygen in water vs air and the structural properties of water and breathable Oxygen. Its just for demonstration and the proportions are not to scale.
So, to answer our question, we did not evolve to breathe under water because the amount of Oxygen in water is just not sufficient to meet our Oxygen demands and thus nature did not take that route.

This explanation however begs the question as to how aquatic life, sometimes bigger than humans, is able to meet its Oxygen demands under water. The answer lies in the fact that most aquatic animals are cold blooded whereas humans are warm blooded. Since, cold blooded animals do not regulate their body temperatures from within like we do, the Oxygen demand of those animals is much lesser compared to warm blooded animals and thus are able to survive in an environment with limited Oxygen supply. 

It is interesting to note that aquatic warm blooded animals do breathe from air (by coming to the surface for each breath) because the Oxygen available in water would not be sufficient to meet their demands.

10 popular myths busted

The advent of social media has helped humanity in more ways than we can count and one of them is decentralizing media. These days anyone with an internet connection can reach thousands of people. However, the flip side is that it has become so much more difficult to tell facts from myths because the sources of our information are much less reliable than they were in the olden days. 

Our Facebook and Twitter feeds are filled so many 'facts' simply shared by gullible friends in most cases and sometimes created by those mischievous people in our circles.

This post tries to call out 10 such myths that have been popular on social media or otherwise. So lets start myth busting - 

1. There are more people alive today than have ever died.
Humans have been here for more than 5 million years and even if we were to consider just the modern humans, we have been around for more than 200, 000 years. Even modest estimates of all humans that ever lived is about 100 billion. 

2. Bulls hate red color.
What makes this myth interesting is the fact that bulls are actually color blind. The bulls in bull fighting are naturally aggressive (only the aggressive ones are chosen), are physically tortured before the fight begins and flapping cape is quite irritating for bulls. It is for these reasons that bulls get aggressive in bull fighting and not because of a red colored cape. 

3. Blood is blue until it touches air.
This doesn't even deserve an explanation. Blood is always red. 

4. Gum takes seven years to pass through your body.
This myth probably originated to keep kids from swallowing gums. The fact though is that gums are mostly digestible like every other food and pass through the body in a normal amount of time. 

5. The "Taste map" on your tongue.
This one is particularly interesting because it has been part of science text books across the globe a couple decades ago. It claims that certain parts of the tongue are responsible for certain tastes (Ex: the tip of the tongue for tasting sweet). However, it was later proven that all parts of the tongue are capable of sensing all tastes albeit with different sensitivity. 

6. That cracking your knuckles causes arthritis.
It was never proven so.

7. Your ring finger is the only finger with a vein that connects to the heart.
So, where do the remaining fingers get their blood from? Probably those who do this propaganda have another blood pumping organ in their body the remaining fingers are connected to. This myth sounds romantic though. 

8. Men think about sex every seven seconds.
Its amazing that there are people who believe this and even worse, there are men who believe this. Sure men are sex maniacs (kidding!) but seven seconds? Really? I believe even once every seven hours would be an exaggeration.

9. Humans use only 10% of their brain.
Unfortunately this is not just a random myth but false propaganda by companies trying to sell some brain-boosters. Next time someone uses this 'fact', its time to ask for some scientific evidence. While it is true that we don't use all of our brain at any given time, we do use 100% of our brains over the course of a few waking hours. 

10. God exists. 
Yes, it is one of the biggest myths ever. May be God exists, but there is no evidence and hence a myth. 

A general advice would be to take anything you find on the internet with a grain of salt (including this blog). Be skeptical. Always look for the author's / organisation's credibility before believing in what they say. A simple rule of thumb would be to not believe anything apart from cat videos that appear on Facebook / Twitter feeds without due research.

Do you know any such popular myths? Please let us know in the comments.

Do we need the outer ears?

As you might already know, most of the machinery that helps us hear sounds is inside our heads (called inner ear) and is not visible externally. People who lose their outer ear do not lose the ability to hear. 

Human outer ear
So the obvious question. Why do we even have these cartilage appendages on either side of the head. If at all they look like they are going to block the sounds coming from behind us!

Turns out there is a good reason why we have external ears (also called pinna). The external ear is this carefully designed sound booster that gathers the sound waves that fall on it and focuses them onto the ear drum via the ear canal. One can imagine what would happen if we were to not have outer ears and just had a hole. The sound waves would pass right by us without ever reaching the inner ear (except for those sounds that come from the sides and thus manage to reach the inner ear).

Studies have shown that the outer ear is a sophisticated piece custom made for humans. For the uninitiated, humans can only hear frequencies in the ranges between 20 Hz - 20 kHz. Of this, almost all of human speech falls in the range of 2 kHz to 4 kHz and turns out, our outer ears are especially good at amplifying sounds in this range. They can selectively boost the frequencies in this range up to 100 times and not boost other frequencies so much before sending them down the ear canal. Kind of like natural noise cancellation technology.

So there you go. Now you know the outer ears are not just girls to tuck their hair in but has a bigger nobler purpose. 

One sad thing though is that humans are among the few animals that can't voluntarily move their outer ears. Most other animals can move their outer ears to focus on the sounds from whichever direction they want to listen to. However with practice it is possible for humans as well to move their ears a little bit though. Can you move yours?


Why are planets spherical?


We take it for granted, but why are planets spherical and not rectangular? Or cylindrical? Not just the planets but most celestial bodies are spherical and any curious kid would like to know the simple science behind this.   

Turns out, it is because of gravity. Planets and stars have a very strong gravitational pull because these things are huge, like really huge. This gravitational force tries to pull pretty much everything to the center and as you can imagine, if everything needs to be as close to the center as possible, it has to be a sphere.

Cubical Planets (Courtesy: AAM)
An example might help convince ourselves. Lets say we set out to construct a tall building. To keep it standing, we need to build a very strong foundation with pillars supporting the structure to counter act the gravitational pull. If the foundation is not strong enough, gravity would pull the building down and bring it closer to the center of the planet (in other words the building collapses). This is pretty much what happens on a much larger scale with all planets and stars. Lets suppose that a planet started off as a cube. Now the corners of the cube can be seen as really tall buildings (a rock building hundreds of miles long). No foundation can ever be strong enough to support such a tall building and gravity would eventually pull these corners towards the center thus converting the cube into a sphere. 

An interesting thing to note here, is that not all celestial bodies are spherical. The smaller ones like asteroids which only span a few miles remain in various bizarre shapes because the gravitational pull within these bodies is not strong enough to overcome the mechanical strength. Below is an image of Phobos, the moon of Mars. It is not spherical because it does not have enough gravitational pull to squish itself into a sphere. 

Phobos: Mars moon is not big enough and thus doesn't have enough gravitation pull to be squished into a sphere
Image Courtesy: NASA
Final note, a little off topic - planets are almost spheres but not exactly spherical. They bulge a little bit at the center (the equator) which is believed to be because of the spinning.

Did the simple science for kids in this post make you a little smarter?
If you have any ideas / questions you want to learn about, please leave a note in the comments section. 
Would be glad to know if there are any inaccuracies in the article.