Increasingly during the past 20 years intellectual development has been recognized as the central purpose of education. The Piagetian model of intellectual development tells us that each student must engage a subject in a manner appropriate to his or herpresentstage of development if he or she is to advance to thenextstage of development. When applied to college physics teaching, this theory implies that the large fraction of introductory physics students who are at the concrete operational stage of development must observe physical phenomena directly while they themselves are manipulating the equipment. Only in this way can they progress to the formal operational stage that characterizes professionals in the field.