Corrected sentences:
A hot-air balloon floats because heating the air inside makes the air lighter. If the air cools, the balloon slowly comes down.
Why I changed them:
- Pronoun clarity: saying "the air" makes it clear what becomes lighter (instead of the vague "it").
- Simple cause-and-effect: "If the air cools" is easy to understand for a child.
- Punctuation: both sentences are short and end correctly with periods.
Great job — your idea was correct! I just made the wording a little clearer.