The overhead projector is an excellent tool for teachers at both the high school and college level. Teachers often use it to display class notes as they monitor students' actions and reactions to the concepts being presented and discussed, to display diagrams and figures too complex to draw on the chalkboard, and more recently to display computer images through an LCD panel. This author contends that the overhead is still not used to its maximum potential; that it can also be used as a means of projecting simple demonstrations of biological principles. Using the overhead to display demonstrations provides all students in the class the opportunity to see the demonstration regardless of their seat location. It also prevents congestion in the demonstration area as students move to gain a better vantage point. This article describes how the overhead projector can be used to demonstrate endocytosis and exocytosis, mitosis and meiosis, protein configuration, and relative sizes of objects such as egg and sperm. The demonstrations take less than five minutes and also reduce the amount of time needed to verbally explain these biological principles. (Contains 6 figures.)