Original: Warm air inside the hot-air balloon makes it lighter, so it floats. As the air cools, the balloon slowly comes down.
Corrected: Warm air inside a hot-air balloon makes it lighter than the air around it, so it floats. When the air cools, the balloon slowly comes down.
Note for a 6-year-old: Warm air is like a little push that helps the balloon go up. When the air cools, the push gets smaller and the balloon comes down.