WHAT YOU NEED: 

• Empty 2 liter bottle
• Party balloon
• Large bowl
• Hot water
• Ice water

WHAT YOU DO: 

• Get enough hot water ready for this entire experiment before you start.  Once you start pouring the water, this moves quickly. 
• Fill the 2 liter bottle with hot water, swirl it around to make the bottle hot and pour it out. 
• Refill the bottle 1/4 with hot water and put the balloon over the mouth of the bottle.  You need a secure seal with the balloon.  This is also an opportunity to try this experiment multiple times: try it once filling the bottle 1/4 of the way, try it again filling it 1/2, try again with only a little bit of hot water and notice how this plays out differently each time. 
• While the hot water is heating the bottle, fill the large bowl with ice water.  Not just chilly water – ice cold water.  Bonus points if there is ice floating in your water - that will make your experiment work best!
• Put the bottle into the cold water and watch what happens. 

WHAT IS HAPPENING:

The hot water in the sealed bottle heats all the air in the bottle.  Hot air takes up more room than the same amount of cold or room temperature air because the molecules are more energetic.  Temperature is just a measure of energy, so hotter objects have more energy.  Since these molecules need more room to bounce around, the air expands and fills the balloon! When you put the bottle into the cold water, it cools the air inside the bottle.  Now that same amount of air doesn’t need as much space because the molecules have less energy to bounce around.  The air condenses, and if the difference between your hot and cold is large enough, your balloon may actually be sucked into your bottle! 

DISCUSSION IDEAS:

Have you ever noticed the tire pressure light come on in your car when it's cold outside. Why do you think that happens? 

What other "real life" applications can you think of connecting temperature and pressure?