The air is made up of molecules. When the air is cold, these molecules are closer together. When the air is hot, they move away from each other. So, in the same space, there are fewer molecules in hot air. Hot air is therefore lighter than cold air. This makes it rise and carry up the balloon when the air inside it is heated. The air is heated in the balloon with a burner, which uses bursts of fire to keep the air hot. The fire is fuelled by propane.