Ruby Brown, of Port Chalmers School, asks :-

Why doesn't air go out into space so we can breath on the moon?

Dick Dowden, a physicist at the University of Otago, responded.

Have you ever played with a "bouncy ball"? If you drop one on a hard floor it bounces back almost to the height of your hand again. If you don't catch it, it goes on bouncing for a long time because it doesn't lose much with each bounce. Now air consists of VERY very tiny balls which are perfectly bouncy. They bounce off the floor, the bounce off every object, even one another, without ever losing anything with each bounce -- if the did, all the air would be lying on the ground like sand and we would have nothing to breathe!

If you throw a ball up very hard, it goes up fairly high, but it still comes down again. But if it was perfectly bouncy it could bounce back to that height again and again. The molecules of air (those very tiny balls), are perfectly bouncy and move at very high speed (faster than a jet plane) but they only go up so high. Only the very fast ones get as high as 10 km above the ground so there are not enough of them up there for you to breathe. The moon is millions of times higher than this so it is not surprising that you can't breathe there either.