Thank you for the question! Air inside the balloon will expand (get larger) when heated, and the air will shrink when cooled. However, the total mass (or amount) of air inside each balloon does not change with temperature. This means that heating a balloon decreases the density of the balloon, and cooling a balloon increases the density. Density is the mass of an object divided by it's volume. For example, a water balloon and an air balloon might have the same volume, but the water balloon is much heavier, so it is more dense. An object's density is what determines if it will drop or rise. The hot balloon is less dense than room temperature air, so it rises. The cold balloon is more dense than room temperature air, so it drops.