Category Theory
Category theory is fun, come on guys let's talk about it!
For those who interested but know nothing about it my recommendation is Basic Category Theory, by Tom Leinster, which can be found on arXiv.
My favorite notion is currying, let me explain it.
Give three sets %%%A, B, C%%% and denote the set of functions from %%%A%%% to %%%B%%% by %%%[A, B]%%%, we claim that %%%[[A \times B], C]%%% is actually isomorphic to %%%[A, [B, C]]%%%.
Before I give the proof you can think about it yourself!