Great question! When you blow up a balloon, the balloon stretches.
Sometimes it can be pretty hard to blow up a balloon, because it
doesn't really want to stretch. And if you don't tie it off right away,
it will probably unstretch by pushing air out of the hole. When the air
comes out of the hole, it creates what's called a force. (You can think
of a force as something that pushes. You can tell that this happens
because if you let the air out of the balloon slowly and hold your hand
in front of it, you can feel the air pushing on your hand.)
Well, one of the most important rules about the way the world works
is that "for every force there is an equal and opposite force."
Basically what that means is that if you're pushing on something, it
has to be pushing back on you. (For example, if you lean on the wall,
the reason that you don't fall through it is because it's pushing back
on you.) Another way to think of that is that if something pushes
(applies a force) in one direction, then it will be pushed (feel a
force) in the other direction.
This is exactly what happens when you let go of a balloon.
The air gets pushed one way, so the balloon gets pushed the other way.
(published on 10/22/2007)