Can you sort a stack of pancakes by size, from the smallest on the top to the largest on the bottom, using only a single plate and a spatula, flipping 2 or more pancakes at the top of the stack (possibly the entire stack) with each flip? How many flips does it take?

Join the ranks of the notables who've tackled this problem, including Bill Gates and David S. Cohen (a.k.a. David X. Cohen, writer & producer of The Simpsons and Futurama).

This activity can be done with pencil and paper, or with the same sorts of manipulatives used for the Tower of Hanoi puzzle – basically, almost any stackable objects, of different (preferably graduated) sizes. But for a fun twist, try it with actual pancakes and a spatula! Just remember: In addition to the pancake batter ingredients, and a griddle, you'll need someone at least somewhat skilled in making pancakes. (Though as you'll see when reading the materials, ending up with pancakes a little burned on one side isn't necessarily a bad thing.)

(Thanks to Bob Klein of Ohio University, who presented this problem – complete with pancakes, cooked by his daughter – in the 6 March 2017 session of the Math Teachers' Circle of Santa Fe.)