It is important that students understand the "open-ended" nature of scientific knowledge and the correct relationship between facts and theory. One way this can be taught is to examine a past controversy in which the interpretation of facts was contested. The controversy discussed here, with suggestions for teaching, is "Expanding Earth "versus" Sea Floor Spreading." Although this was a short and limited controversy within the mainstream scientific comrnunity, it has the advantage of having primary sources that are accessible to students to read for themselves. What makes this controversy intriguing is the later conversion of one of the protagonists (Tuzo Wilson) to tectonics. The controversy is framed explicitly in terms of several criteria for agreeing on the optimal theory: it is an exercise in what has been termed "theory choice" by Thomas Kuhn. Framing the controversy in this way can teach students a great deal about the emergence of scientific theories as well as criteria that can be used to judge ideas in a mature fashion.