So, I want to start a mathematics club at my high school. To get people interested in attending some of the beginning meetings, I want to set up posters with either
An interesting/beautiful visual example of mathematics
A nice puzzle or riddle (that students could come to the club to attempt to solve or have the solution revealed for them)
I'm sure that lots of people on this site have great examples that would fit nicely. I've seen some questions like this on the exchange but I'm really looking for something that is
Attention grabbing on a poster and understandable to anyone.
I look forward to seeing the answers!