The beat is the division of musical time into units of equal duration, or, alternatively, the regularly occurring pattern of rhythmic stresses. Whereas the beat must remain constant during a phrase or passage in a work, rhythm is variable. The beat in music is sometimes compared to the regular beating of the human heart and pulse, to the steady ticking of a clock, or even to the continuous dripping of water. Together with the tempo, which is the speed at which a beat occurs, they create a sort of scaffolding on which rhythms can be constructed.