Kindergarten Physics: Air

What you need:

• a glass bottle
• a balloon
• a jar (the glass bottle need to fit in)
• a kettle (to heat water)
• oven gloves

OK, let's start. First, put the balloon on the bottle, like this:

Then put the bottle into the jar:

Heat some water (boiling hot water works best), and fill the jar with it. Hold down the glass bottle, so it doesn't start to float. Wear the gloves to prevent burning your fingers (this step and all following involving hot water should not be done by small children!!).

So what happens? After a while, the balloon gets bigger:

The hot water heats the bottle and the air contained in the bottle. This causes the air to expand (all gases expand when heated and air is a mixture of different gases). As there's not enough room inside the bottle some air leaves the bottle and pumps up the balloon.

OK, now the second part of the experiment:

Remove the balloon from the bottle, still leaving the hot water in the jar:

Put the balloon on again. Now it has its original size:

Remove the bottle from the jar, pour out the hot water, put the bottle back in:

Now pour cold water into the jar. 

After a while the balloon is sucked inside the bottle and is "pumped up inside out".

What happened?

There was still hot air inside the bottle. When the hot water is replaced by cold water, the bottle and the contained air is cooled down as well. The air contracts and now uses less space. There is now some "empty space" left in the bottle, that can be filled by other air. Therefore, air from outside is sucked into the bottle together with the balloon that sits on the bottle, causing the ballon to get pumped up inside out.