If you ask your child how much air can fit in a bottle, she'll probably say it depends on the size of the bottle. True, but there's another factor to consider, and that's temperature. A bottle can hold more cold air than warm air. With this simple activity, your child can see for herself.
• An empty 20-ounce plastic soda bottle, without the cap
• A glass of water (wide enough for the top of the soda bottle to fit in)
• Bubble solution (you can make your own by mixing a few drops of dish soap with N cup of water)
Place the soda bottle mouth down in the glass of water and watch what happens: not much. Dry the bottle and chill it in the freezer for 5 minutes. Once again, place it in the glass. Without being squeezed, the bottle will blow underwater bubbles.
Return the bottle to the freezer for another 5 minutes. Meanwhile, pour a little bubble solution into a shallow bowl. When the bottle is ready, dip the mouth in the bubble solution, then set the bottle upright on a table. A bubble should start to form almost immediately. When the bubble bursts, redip the bottle; it should keep on blowing bubbles until it nears room temperature.
When you put the bottle in the freezer, the air inside it gets cold. That's because air, just like everything else, is composed of molecules (tiny bits of matter) that react to changes in the environment. The colder the air, the closer together the molecules get. Because it's denser than warm air, you can fit more of it in the bottle. When you take the bottle out of the freezer, the air inside it starts to warm up and expand. The bottle can't contain it all. As it escapes, the air pushes against the water in the glass (or, in step 3, the bubble solution on the rim), creating bubbles.