One day while doing a computation on the board in front of my students, I accidentally wrote 1 + 1 = 1.
(No idea why.)
Student: Um, don't you mean 1 + 1 = 2?
Me (embarrassed): Oh right, thanks. [Erases mistake. Pauses.] Wait. Is there a universe in which 1 + 1 = 1?
Class: ... [Stares blankly]
Me: That's not such a strange question. Don't you know that YOU live in a world where 11 + 1 = 0?
Class:... [Jaws drop.]
Me: Yeah! Think about it! 12 = 0 and 13 = 1 and 14 = 2 and 15 = 3... sound familiar?
Class: Military time!
Me: Exactly! [Points to clock on the wall and starts impromptu lesson on modular arithmetic.]
I hope at least one student was convinced that math is cool.
'Cause I'm convinced that making mistakes isn't always a bad thing!