Blow up a balloon with a pop bottle

What Happens When You Heat Air?

This is a neat trick to teach your kids about what happens when air is heated or cooled. You just need a few simple things to blow up a balloon without using your breath!


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  • A balloon
  • A recycled 2-liter pop bottle or water bottle
  • Kitchen sink OR a bowl of hot water and a bowl of cold ice water


  1. Attach the balloon to the top of the pop bottle. The balloon will be deflated.
  2. Turn on your sink and make the water as hot as it will go (WORK WITH A PARENT)
  3. Place the bottle under the water and watch what happens to the balloon (it will INFLATE)
  4. Next, turn the water to cold and put the bottle back into the water (it will DEFLATE)


When you attach the balloon to the top of the bottle, you are trapping the air inside. We never see the AMOUNT of air change in the bottle but the air molecules act differently based on the temperature! When you heat up the air inside the bottle (using the hot water) the molecules move faster and spread apart.

The warm air in the bottle is expanding the balloon will blow up because the warm air takes up more space. When you put the bottle under cold water, the same amount of air inside the bottle cools down and contracts. It takes up less space and the balloon will deflate!