When circles become squares, flat becomes curved, and coffee cups become doughnuts, you know you’re in the weird mathematical world of topology. Sometimes called “rubber geometry,” topology is the study of properties of objects that don’t change even when those objects are twisted, stretched, shrunk, and bent. On a recent full-day field trip to the National Museum of Mathematics in Manhattan, our students played “Topological Tic-Tac-Toe” and enjoyed a guided exploration of the museum exhibits.