Air expands when heated, which makes it lighter than an equal volume of cool air.
The heat for the hot-air balloon comes from a burner that uses propane, a safe and inexpensive gas. The burner produces a flame that reaches up into the bag. The bag of hot-air balloon is made of nylon or polyester. To carry two person, the bag must have a volume of about 1,700 cubic metres. The bag normally has a circular rip panel at the top.