Inspired a while back by Chris Lusto’s “What’s in a Circle?” blog post, I’ve been saving this question for this week specifically, since it’s Pi day this Friday. So I asked students, “What is a circle?” I had them write a definition on their own, then share with their groups, then pick the “best” definition to share with the class. Finally, I had each group write a set of instructions for drawing a circle, for someone with no idea of what a circle is or the concept of what “round” means. It was great. Tons of struggle and debate over the best way to do this. Then I had groups rotate through the room and follow other groups’ instructions, which produced plenty of odd shaped figures, none of which were circles. It’s always great to see students struggling with ideas that seem very straightforward on the surface, but have much more to offer when you dig in. Great lesson.