The basic principle behind hot air balloon physics is the use of hot air to create buoyancy, which generates lift. A hot air balloon consists of a large bag, called an envelope, with a gondola or wicker basket suspended underneath. A burner (with power typically of several megawatts) sits in the basket and is used to heat the air inside the envelope through an opening. This heated air generates lift by way of a buoyant force. The figure below shows a typical burner.