Check out this website by Professor Francis Su!
Math Fun Facts are ideas and puzzles that will change the way you think.
This archive is designed as a resource for enriching your math courses and nurturing your interest and talent in mathematics! Each Math Fun Fact is a math puzzle or short article that contains a cool mathematics idea. You’ll can learn about the mathematics of things like card shuffling to poker to computer vision to fractals to music, just to name a few. This makes great enrichment material for gifted math students or problem-solving groups.