Why does hot air rise?
For grown-ups: the science
Hot air rises because warming makes it spread out, so it is lighter than cool air around it.
Heating a parcel of air increases molecular motion, causing it to expand. At roughly the same pressure, its density decreases, so buoyancy from denser surrounding cooler air pushes it upward; gravity acts on both parcels, but the denser air provides a greater downward weight difference.
What about...?
Ask your child's own question
Transcript
A warm balloon floats above a campfire. Why does hot air rise?
Air is made of tiny pieces we cannot see. When air gets warm, those pieces move around faster.
The moving pieces spread farther apart. Warm air takes up more room than the same amount of cool air.
That makes warm air lighter for its size. Cooler, heavier air slips underneath and nudges the warm air up.
So hot air rises because it spreads out and becomes lighter than the cool air around it.