Wonderful choice, you opened the door to...
GRAVITY!! This is the most commonly known and observed force. But you say, "What exactly is gravity?"
Gravity is an attractive force that draws masses to each other.
"What?" says you, "I thought it made things fall!"
Aha... so it does. Let's see...
You are a small mass compared to the earth. Why do you fall back on the earth after you jump? Well, the smaller mass is always attracted more to the larger mass, this means you (the smaller mass) are attracted to the earth (the 
larger mass). The larger the mass, the more you are attracted to it.
"Is the larger mass attracted to me?" says you.
Very good question! Yes, the larger mass is attracted to you, but because the mass is so large compared to your mass, it's movement is not easily detectable.
"Okay, I see that, but astronauts on the moon don't feel gravity."
Aha! But there is gravity! Gravity is everywhere because it acts between and among all different masses! Gravity's range is infinite - it reaches everywhere! Therefore, astronauts do get affected by gravity on the moon, the earth, and everything in the universe!
"Why is the gravity different on the earth than on the moon?"
Great! Now your thinking like a scientist...wanting to know why something happens. Well, let's see if you can figure this out with some examples...
On Jupiter (a very massive planet) you weigh a lot, but on the moon (which is a small mass compared to Jupiter or Earth) you weigh a lot less! And on Earth (a mass in-between Jupiter and the moon) you weigh normal. (Distance is also a factor in gravity. If you are far away from a mass, then it's gravity doesn't affect you as much as when you are on top of the mass. For example,
you don’t feel Jupiter’s gravity very much on Earth, but when we actually go to Jupiter, you will feel extremely heavy.
That's right! If the object you are standing on magically increases its mass, your weight will increase. That is, more massive planets have a larger gravitational force! This means that astronauts can do lots of different things on the moon than on the earth because the gravity is not as strong on the moon as it is on Earth. The moon is not as massive as the earth. Wow!
"Okay, now that I understand gravity, why does it happen?"
On your planet, Isaac Newton discovered gravity. He said that gravity is the force that attracts masses to one another. Since then, the idea of gravity has progressed. There is a hypothesis that states that the attraction between masses (gravity) exists through the exchange of gravitons.
"Cool, what are gravitons?"
Gravitons are particles that every mass gives off. Gravitons are very interesting particles because they have no mass! Gravitons are exchanged between everything—this means you are exchanging gravitons with the sun and even your friends! Because gravitons have no mass, no one can see them. But...I have created a pair of special glasses that you can use to view the gravitons in this picture!
Click on the glasses to see the picture! 
"That's it... You mean there's nothing more about gravity than gravitons?" you say disappointedly.
Actually...there's more...
"More?"
Yes! Gravity can act on light! Why? Well... it has been observed in nature that the path of light can be bent in vacuum if the light rays are near a very massive object called a black hole. A black hole has such a strong gravitational attraction that some scientists speculate that the whole universe revolves around a large black hole!
"But, why are they really special?"
They are special because their gravitational attraction even draws light in from nearby stars!
"Whoa! How can the direction of light be changed?"
Light is also made of a hypothetical particle called a photon. Photons are a lot like gravitons, they have no mass. The black hole distorts space and time thereby capturing the photons passing next to it. The distorted space looks sort of like the vortex twister you see when bathtub water drains from the tub. At the end (the tip) of the vortex is where you would find the black hole itself. Similar to a soap bubble following the vortex path of the water in the bathtub drain, the photon must follow the distorted path of space generated by the black hole.
"Wow, gravity is so cool!"
