You and three other people draw cards from a deck and hold them to your foreheads (so that everyone else can see your card, but you can’t). You all guess the suits of your own cards simultaneously. You all win if *at least one* person guesses correctly.
You can all agree on a strategy before drawing cards. What do you do?
(Warning: Spoilers! The post is mostly about finding weirder / more general solutions.)
A very incomplete introduction to group theory
Strong opinions about algebraic structures
The worst theorem in math
A stained glass window