It makes sense that a balloon filled with a lightweight gas such as helium would float when surrounded by heavier air.....

But why does filling a huge balloon with hot air also make it float? This resource from the NOVA website offers a series of interactive activities that describes and illustrates the chemistry (mass, volume, density) of what happens inside hot air balloons.

Go through this NOVA site and then summarize the key points learned.

Equal volumes of materials can have different masses. Density is the relationship between material's mass and its volume. Helium has less density than air. When heated, hot air balloon's total mass of air will decrease yet, it's volume will stay constant.