We're going to cut up a balloon to use for this experiment. Uninflated balloons are so small, all we need to do is cut the ends off, then make one cut from the top to the bottom. Then what we have is a rectangle of thin rubber.

Using a rubber band, we'll stretch the piece of rubber over the mouth of an empty bottle. The balloon piece has become a membrane. Now, the bottle will be placed near a source of heat. It can be near a heat duct or in a pan of very warm water.

Watch what happens. You think it isn't much of a biggie to put an empty bottle into hot water, but you may get a surprise!

Look at that, will ya? Something is pushing the membrane outward. Do you suppose there is a genie in the bottle, trying to get out? Probably, all that is inside the bottle is air. When it is heated, it expands. The "genie" is warmed air, pushing on the only surface of the bottle that is "pushable."

So what happens if we cool the bottle? Let's put it into a bowl of ice cubes and find out what it will do next. That membrane sinks a bit, because the chilled air contracts. Maybe the warm air inside the room pushes down on the membrane while the chilled air is contracting.

We could do this in a little different way by putting several different-sized bottles into the refrigerator for an hour or so, and then, stretch a membrane across the tops of each one. Just being brought out from the fridge into the classroom may be enough of a change of temperature to move the membrane.

Or, we could do it backwards by heating bottles before putting on the membranes, and watch them as the air inside returns to room temperature.

Does the size of the bottle make any difference? Or the size of the mouth of the bottle? Does more air inside a big bottle with a narrower mouth push the membrane out farther than the air inside a small bottle with a wide mouth?