I'm happy when my students learn good math in my classroom. I love solving real problems with my students and having them go out into the world with a better understanding of why things work. Even more rewarding is when they tell me how they solved a problem that occurred in their life with mathematical reasoning.
But what I get the biggest kick out of is students using math with each other for no good reason other than they think it's cool. Monday night in my cryptology class, we talked about Bacon's bilateral cipher and all the students got charged with the idea that they could hide a message in seemingly innoculous text just by converting letters to numbers in binary and bolding the appropriate characters in the regular text.
After a long day at work yesterday, I got home and checked my Facebook. I saw one of my students trying to bold his status and failing.* So I left him a little message, letting him know in Bacon's code that I was sorry he couldn't bold. (I just used uppercase and lowercase letters instead. More obvious but usable.) Next thing I know, other students are leaving messages in Bacon's cipher and a student that isn't even in the class figured out the cipher just because he didn't want to be left out!
When I finally logged out of Facebook, I was nearly radiating from all the geekiness.
*Yes I currently friend students. You can scold me on that later. I go back and forth on the reasonableness of that decision daily.