# Water Balloon in a Bottle

This fun science trick is actually an awesome demonstration of air pressure.

If we asked you to blow up a balloon inside of a bottle or, better yet, fill that balloon with water, you’d probably think that it’s no problem. Think again! The Water Balloon in a Bottle experiment will show you just how powerful air pressure actually is, and you’ll get to play some fun tricks on your friends. Having fun with friends and learning along the way? It’s a hands-on science two-for-one!

## Experiment

### 1

Put the balloon into the bottle, wrapping the open end around the mouth of the bottle.

### 2

Take a deep breath and try to blow up the balloon. No matter how hard you blow, it just doesn’t blow up!

### 3

Now, poke a hole in the bottom of the bottle using a tack or pushpin.

### 4

Take another deep breath and try blowing up the balloon. Success!

### 5

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!

### 6

Take your finger off of the hole in the bottle and the balloon will deflate.

### 7

Now it’s time to make the water balloon! Blow up the balloon as directed in steps four and five, then cover the hole in the bottle with a piece of duct tape so that the balloon stays inflated. Make sure that the tape will be able to hold for a while.

### 8

Fill the inflated balloon with water and tie off the balloon. You’ve achieved the water balloon in a bottle.

### 9

If you want to play a trick on your friends, blow up the balloon and fill it with water as described in steps seven and eight.  This time, however, don’t tie off the end of the balloon 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

Although the bottle appears empty, the first thing to remember is that it is actually filled with air.  Before you poke a hole in the bottle, the only way for the air to escape is through the mouth of the bottle. Once the mouth is covered with the balloon, the air is trapped inside.  When you try to blow up the balloon, it 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 because there is no air rushing back into the bottle to push on the balloon. When you take your thumb off the hole, outside air flows back into the bottle as the balloon collapses, releasing the air (or water) that was inside of it. Because of the elasticity of the rubber or latex, the balloon shrinks to its original size as the air or water rushes out the top of the bottle.

## Take It Further

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.

## Science Fair Connection

Inflating a balloon inside of a bottle is pretty cool, but it isn’t a science fair project.  You can create a science fair project by identifying a variable, or something that changes, in this experiment.  Let’s take a look at some of the variable options that might work.

• Change the number of holes to see how it changes the speed of inflating or deflating the balloon.
• Change the size of the holes to see how it changes the speed of inflating or deflating the balloon.
• Is there a difference with the speed of deflation when the balloon is filled with water vs. air? What about other substances?

That’s just a couple of ideas, but you aren’t limited to those! Try coming up with different ideas of variables and give them a try.  Remember, you can only change one thing at a time.  If you are testing different sizes of holes, make sure that the other factors are remaining the same!