Hot Air Rises
(Thanks to American Aircraft Modeler)
You can make your own tiny hot air balloon to see how hot air rises and then cools down and sinks again. Take a Kleenex tissue or some toilet paper and glue the edges to a circle of light string with tiny bits of glue (too much glue will make your balloon heavy). Hold your balloon over a heat source - the burner on an electric stove, or a hairdryer, or a candle if you are very careful not to catch it on fire. When it heats up, let it go and it should rise. But after your balloon rises away from the heat, the air inside will cool off and it will come back down.
You can also make a bigger, more impressive balloon, but you'll have to buy some tissue paper. Get big sheets of tissue paper, the kind you use for wrapping presents (not wrapping paper though!). Glue them together to make longer sheets about seven feet long. Glue eight of these long sheets together in a circle to make a balloon about five feet tall (and glue their tops together too. You might want to cut them into triangles near the top so they will fit together better into a balloon, like the sections of an orange).
After the glue is dry, take your balloon outside to a parking lot and put some cotton balls and alcohol in a tin can and light it (if you are a kid, get adult help with this!). Hold your balloon over the can until it is hot enough to rise into the sky. Don't add more alcohol to the can once it is on fire, or you'll burn yourself. If the balloon catches on fire, just let go of it and let it burn up. It will go out by itself.