What you will need:
- Wide mouth bottle
- Water balloon(s)
- Straw or tube
- Adult supervision
Fill up a water balloon to about tennis ball size and tie it off. Now, try putting the water balloon inside your bottle. You should notice that as you try to put the water balloon in the bottle, it will begin to go in but then stops suddenly. No matter how hard you try, it is almost impossible to get the balloon completely in the bottle.
Pull the balloon out and place a straw or tube in the bottle so that it goes all the way in the bottle with the other end sticking out of the bottle. Now try putting the water balloon in the bottle again, making sure that both ends of the straw are not covered or plugged by the balloon.
Once you get the balloon in the bottle, try pulling it out WITHOUT the straw in the bottle. Now try it again with the straw in place as before.
What is going on?
Even though the bottle looks empty, it is full of air molecules. Air takes up space and has 14.7 lbs of pressure per square inch. As you try to put the balloon in the bottle, air gets trapped in the bottle and cannot escape since the balloon has sealed off the opening. As you push the balloon in the bottle, the space inside the bottle is getting smaller. As this space gets smaller, the air molecules are getting pressed closer together creating an area of higher pressure that will push back on the balloon and not allow you to push the balloon in the bottle. When you place a straw in the bottle, the air is then allowed to escape out through the straw which will then keep an equal amount of air pressure both inside and outside of the bottle and allow the balloon to be placed in the bottle. The same principle takes place as you try to remove the balloon from the bottle without the straw except that the space is increasing in size creating an area of lower pressure which then pulls back on the balloon as you try to remove it. Adding the straw again allows the air to flow into the bottle as you pull the balloon out.