SCIENCE EXPERIMENT AT HOME
Fill a plastic (PET) bottle with hot water (NOT boiling water -- see if you can work out why I said that!) and fill a bowl with cold water. Let them sit for one minute, then empty the bottle quickly. Stretch a balloon over the open end of the bottle and push the bottle down into the cold water. What happens? Why?
Why does the balloon inflate into the bottle?
The warm water heats the bottle which, in turn, heats the air inside the bottle after the water is poured out. When the bottle is placed in the cold water, the air inside cools and contracts, causing outside air to be drawn in, pulling the balloon in and inflating it inside the bottle.
Try sitting the bottle back in the hot water again.
Another way to try this uses a balloon, a plastic drink bottle, some plastic tubing, rubber glue, a bicycle pump with the valve of the piston reversed (to suck instead of blow), and a drill.
You will have to drill a hole in the side of the bottle near the bottom, choosing a size which will just take the plastic tubing, and then glue the tubing in place. Hook this up to the pump, or use the connector from an old inner tube, but with the valve removed. Then push the balloon down through the neck of the bottle, and reverse the neck of the balloon down over the outside of the bottle neck. When air is drawn from the bottle, atmospheric pressure is greater, and the balloon "blows up" inside the bottle.
The main problem will be finding a bicycle pump which allows the valve to be reversed. The modern plastic variety does not allow this, so look for and old relic, with a metal barrel which can be unscrewed.
Yet another variation: Try shaking a small amount of very hot water in a bottle to heat the air inside, then quickly fit a balloon to the neck. Then wait a few minutes for things to cool down before you sit the bottle in an ice bath. Some of the pressure inside the bottle will have been a result of water vapour, which now condenses.