The easiest way to think of gravity is that it's the thing that makes you stick to the Earth. It's like a giant hand that reaches up from the ground, grabs your ankle and holds you down. Gravity affects the amount of force that you exert on the ground. It's like you are pushing on the ground!

Have you ever wondered why you don't just float off into space? It's because of gravity! 

When you see movies with astronauts in space and they are floating all around, it's because they aren't close enough to the Earth (or to another planet) to get pulled down by gravity. When you see the astronauts on the moon (or on another planet in a movie), they are getting held down by that thing's gravity. In fact, it's the Earth's gravity that holds our moon in it's orbit... Without gravity, the moon would just go flying out into space!

Now that you know what gravity is, we can tell you what weight and mass are... and what's the difference between the two. We'll bet a lot of grown-ups don't even know this!

Mass is just the amount of "stuff" you are made of! No matter where you go in the Universe, you are still made of the same amount of stuff, so your mass is always the same. (Unless, of course, you grow. Then your mass increases.)

The force of gravity (the big white arrow in the picture above) that you push on the ground with is called weight. You might weigh 66 pounds on the Earth... But you would weigh 1/6 of this on the moon (only 11 pounds) because the moon's gravity is 1/6 as strong as the Earths. If you go to our Solar System lesson, you can find out how much you weight on each planet, the Sun and our moon!

This is a really important thing. It's so important we call it "The First Law of Motion!" It's a pretty easy idea... 

If something is moving, it will keep moving until something stops it. If something is not moving, it will just sit there until something comes along to move it. 

Here is the official, scientific way to say it: An object in motion will continue to stay in motion unless acted upon by an outside force. An object at rest will continue to stay at rest unless acted upon by an outside force.

Here's a good example:
Picture yourself racing down the sidewalk on your blades. If you pushed off just once and coasted, how long would it take you to stop? Would you stop a lot faster if you were on asphalt instead of smooth cement? You sure would! That's because there's a lot more friction to slow you down on the asphalt.

Now, what if you were using ice skates on ice? It would be nice and smooth - less friction - and you'd coast a lot longer right?

Well, there's no friction in space... If you were coasting out in space, there'd be nothing to stop you and you'd just keep drifting! Unless something like a comet came along and stopped you. Get the idea? That's inertia!

What if you put a book on a table... It would just sit there unless you moved it. You could pick it up... or push it... or lift one side of the table so it would side off. That's inertia!