Heat causes matter to expand. Gases, including air, expand the most when heated. Warmer air is lighter than cooler air because warm air molecules are more spread out. The burner in a hot air balloon heats the air inside the balloon. The hot air inside the balloon causes the balloon to float upward into the heavier, cooler air outside the balloon. When the burner is turned down, the air inside the balloon cools off, and the balloon sinks.