A hot air balloon works by applying these simple principles to air.
First, a fan is used to blow air into the balloon, but not enough to fill it up completely. Then, a burner is used to heat up the air inside of the balloon. When this happens, the air molecules in the balloon speed up and move farther apart. This causes the heated air to occupy a larger volume than it did before it was heated.
Because the heated air now occupies a greater volume, but still has the same mass, we know that its density has decreased. How do we know this? Remember that density is a measurement that compares the mass of a substance to the amount of volume that it occupies.
Hot air balloons float because the volume of the balloon is so large that the entire balloon, including the basket and the people is less dense than the surrounding air.