Geoffrey Twyman, age 7, of Calgary, Alberta, Canada, for his question:

What makes a rocket fly?

The secret is a special kind of push. In a small way, you can make it work with a toy balloon, though you can't actually see it work. This is because the push is done by moving air and air is invisible. Blow up your balloon and hold onto the string. Now cut the knot that ties it  ¬and let go. As the air spurts out, the shrinking balloon takes off with a swoosh. Actually, the air that escapes through the hole pushes the bal¬loon in the opposite direction.

A rocket burns fuel that creates a spurt of hot, expanding gases. This escapes through a nozzle in the back of the rocket. The hot gases need more room to spread out and expand    and they push very hard to get it. And the rocket happens to be in the way. As the hot jet escapes through the back, it uses its pushing power to push ,t he rocket in the opposite direction    which is forward.