Nowadays, the hot air balloon comes in many shapes and forms, from rocket ships to sausages and teddy bears. The passengers and loads are usually carried in a gondola which has a source of heat, in most cases- an open flame. The heated air then travels inside the balloon which makes it lift and buoyant due to its density being lower than the air outside the balloon. Unlike gas balloons, the envelope of the hot air balloon doesn’t have to be hermetically sealed due to the gas not being kept under pressure, but instead it having the same pressure as the surrounding air.
Today, most of hot air balloons are made of nylon, and the base of the balloon (the inferior part which is closer to the heating source) is made out of a heat-resistant material, such as Nomex.