Carnival of Mathematics (inaugural edition)
The first ever Carnival of Mathematics appeared yesterday on Abstract Nonesense. Alon did a nice job of sorting out some nice ones, and arranging them from most elementary to most complex. My quadrilateral post, by the way, is near the middle.
The Carnival of Mathematics covers pure math, applied math, school-level math, math education, debunking bad math, math history, math trivia, and even puzzles/problems. Go take a look, you’ll likely find something of interest.