EXPERIMENT

Let's start by trying to blow up a balloon inside of a one liter bottle. No problem right? Put the balloon into the bottle, wrapping the open end around the mouth of the bottle.
Take a deep breath and blow up the balloon. You can't do it? Oh no! What's going on here?
We'll answer that question in a bit. For now, poke a hole in the bottom of the bottle using a tack or pushpin.
Take another deep breath (remember how hard… er… impossible it was to blow up the balloon last time) and try blowing up the balloon.
That was easy! While the balloon is still inflated, cover the hole in the bottom of the bottle with a finger and take your mouth off of the balloon. It stays inflated!
Take your finger off of the hole in the bottle and the balloon will deflate. Wow!
Now let's add some water.
Blow up the balloon and cover the hole in the bottle with a piece of duct tape. Make sure that the tape will be able to hold for a while.
Fill the blown up balloon with water and tie off the balloon. You've achieved the water balloon in a bottle.
You were promised a trick to play on your friends, so here you go!
Once you've blown up the balloon and filled it with water, leave the balloon open, and cover the hole in the bottom with your finger. Show a friend that you've successfully put a water balloon in a bottle. When they attempt to check whether or not there is water in the balloon, let your finger off of the hole in the bottle. The water blasts out and gets them all wet!

HOW DOES IT WORK?

The balloon won't inflate much the first time because the bottle is already filled with air. There's no room for the balloon to expand inside the bottle. However, when you punch a hole in the bottle, the air molecules in the bottle have an exit. They're pushed out as the balloon fills the space inside. As long as you plug the hole, the balloon stays inflated. When you take your thumb off the hole, outside air flows back into the bottle as the balloon collapses. Because of the elasticity of the rubber or latex, the balloon shrinks to its original size as the air rushes out the top of the bottle.

Suppose your thumb gets tired while the balloon is inflated. Put a cap tightly on the bottle and remove your thumb. For the air to flow, both holes have to be open. How would more holes or even one large hole change the speed of inflating and deflating the balloon? What would more or bigger holes do to the stream flowing from the water-balloon-bottle? Try it out! Balloons and bottles make a great science combo!