Much of human learning throughout the lifespan is achieved through seeking and generating explanations. However, very little is known about what triggers a learner to seek an explanation. In two studies, we investigate what makes a given event or phenomenon stand in need of explanation. In Study 1, we show that a learner's judgment of "need for explanation" for a given question predicts that learner's likelihood of seeking an answer to this question. In Study 2, we explore several potential predictors of need for explanation. We find that the need for explanation is greater for questions expected to have useful answers that require expert understanding, and that "need for explanation" can be differentiated from general curiosity.