Fig. something that motivates someone; something that makes someone behave in a certain way. (Fig. on what makes something tick.) William is sort of strange. I don't know what makes him tick. When you get to know people, you find out what makes them tick.

Fig. the sense or mechanism that makes something run or function. (With reference to the ticking of a clock representing the functioning of the clock.) I don't know what makes it tick. I took apart the radio to find out what made it tick.