Here are four simple one-sentence ways to say it:
- When the air inside a hot-air balloon gets hot it becomes lighter so the balloon floats up, and when the air cools it becomes heavier so the balloon goes down.
- A hot-air balloon climbs because heating the air inside makes it lighter, and it drops when that air cools and gets heavier.
- If the balloon's inside air is warmed, the balloon rises; if the air cools, the balloon falls.
- Warm air inside a balloon makes it lift off into the sky, and cold air makes it sink back down.