This (or, even better, a physical Spirograph, together with a document camera) is spectacular for teaching mathematics. In my calculus class, we wrote down par

This (or, even better, a physical Spirograph, together with a document camera) is spectacular for teaching mathematics. In my calculus class, we wrote down parametric equations defining the curves you get. In my abstract algebra and number theory classes, I got the students to figure out the rule predicting how many loops the figure will have.