Why doesn't the moon fall down?
Now there was a very clever man about 400 years ago called Sir Isaac Newton, who, in between discovering lots of mathmatics, writing about theology and inventing the wibbley edges on pound coins, was thinking about this sort of problem.
The first question he asked was what do we mean by fall down? If falling down means that everything falls in the same direction, and the world is round, things on the bottom of the Earth wouldn't stay there very long..
So obviously falling down means something a bit different! What happens is that everything falls towards the centre of the earth, (that way the kangaroos don't have to be tied down!). Now Issac started thinking about a cannons, now he knew that the faster you fired the cannon ball the further it went.
Now Issac, possibly sitting in a pub, thought what would happen if we had a really big cannon - no a really, really, really, reeeeaaally big cannon, and put it on top of a mountain (Oh and pretended there was no air to slow it down), how far could you get it to go?
So if you make the cannonball go fast enough it keeps falling all the time, but it never falls quite fast enough to actually hit the ground. This is called an orbit, and how astronauts, satellites, the moon don't crash into the earth.
So the moon, it is actually falling down all the time, just it always misses the earth and always will.
If you want to experiment with planets have a go with a Gravity Simulator.