Teaching probability and statistics is more than teaching the mathematics itself. Historically, the mathematics of probability and statistics was first developed through analyzing games of chance such as the rolling of dice. This article makes the case that the understanding of probability and statistics is dependent upon building a “mature” understanding of common random phenomena such as the rolling of dice or the blind drawing of balls from an urn. An analysis of the verbalizations of 24 college students, who interact with random phenomena involving the mixture of colored marbles, is presented, using cognitive schema to represent the subjects’ expressed understanding. A cognitive schema representing a mature understanding is contrasted to a diversity of observed immature understandings. Teaching to explicitly build the mature cognitive schema is proposed.