The geometry

Of miniature golf. Obviously.

Of packing for a week-long trip with only a teeny-tiny carry-on (which, by the way, is the only type of carry-on most people should have; no one likes the guy who takes forever to stuff a large case into the overhead compartment just slightly bigger than my car's glove compartment).

Of the pool table.

Of the shapes and physics of the world, natural and man-made.

Geometry is my favorite math subject, usually tied with history of math. Right now, since I'm tutoring a high school kid in geometry, I've rediscovered how much I like proving triangles are congruent for no other reason than to prove two things are congruent.

I would like to find some more geometry puzzles and puzzlers though. The ones I've found online are rather, well, elementary. Which is still good - for a fifth grader, but not a 15-year-old. There's got to be more difficult stuff out there.