Choose the correct words to fill in each gap.

Hot-air balloons
A hot-air balloon ... of three basic parts. The large balloon is ... as the “envelope” and is made of nylon — the same material used to produce kites, sails and sleeping bags. There are burners under the envelope to heat the air inside, and finally, a basket ... the passengers stand. From this ... , the passengers can enjoy the views of the countryside below.
... understand what allows the balloon to lift into the air, you must know about “convection,” or heat transfer. The scientific ... is that by heating the air inside the envelope, you increase the space the air ... . When this happens, some air gets pushed out of the envelope. With less air in the envelope, it ... lighter and, therefore, able to travel up. Once the burner is shut off, however, the opposite happens. Cold air from outside enters the envelope, making the balloon heavier, and it starts to fall.