To blow air through the organ, there are boxes called "wind chests". When the organist is playing, he/she can see a little gauge that tells whether there is enough air. The wind chests can be kept full in two ways. The old-fashioned way is to have an enormous set of "bellows" which are pumped up and down by a person using a large handle. This sucks in air and fills the wind chest. Pumping the bellows of a large pipe organ is heavy work. For this reason, most organs nowadays have an electric motor and a large fan which fills the wind chest.