I've always had a passing interest in mathmatics, and to this date enjoy thinking about some of the paradoxes that arise when you talk about math theory. Just don't ask me to do any of the math!
One of the questions that arises is infinity. Now infinity is a difficult enough topic to grasp, but - as detailed in DFW's Everything and More (a summary of the infinity) - there are even different sizes of infinity. It's a huge mind-fuck of a concept, but a hell of a lot of fun to ponder.
Or to get a cliff notes version, you can check out The Dish's ongoing conversation on infinity here and here.