8th grade geometry. Day 1.
I am part of a wonderful network of teachers, brought together by Henri Picciotto, called Escape from the Textbook. I learned of this problem by one of the members of the group who uses it as a way to introduce the idea of formal proof. It’s what’s for breakfast tomorrow: 1st period of the 1st day of school. Fun stuff, I hope. After a bit of research on Wikipedia, I learned that this problem is called the Mutilated Chessboard Problem and was published in 1958 by George Gamow and Marvin Stern.