Appearing under My favorite math joke is another joke which claims the title of shortest math joke:

Let |epsilon| < 0.

While yours has less characters, this one seems to be simpler, more elegant, somehow; I think, therefore, that it is more deserving of the title.

Note 1: When I first wrote this, I didn't even spot the similar one at the top of the node; I would like to point out, though, that without the absolute value, the joke doesn't really work.

Note 2: People have been asking me for an explanation of the joke. Here goes. The basis of it is that fact that |epsilon| > 0 frequently appears in delta-epsilon proofs, and in other math too I would guess. The pun in |epsilon| < 0 is that, though it looks almost like |epsilon| > 0, which is legitimate, an absolute value cannot be less than 0. That is, |x| means the absolute value of x, which means remove the sign from x, make it positive. Thus while |epsilon| < 0 at first glance (and second glance) looks perfectly normal, after a moment, math nerds notice that it is completely impossible and nonsensical, which to me is the irony which makes it funny. There, I've dissected it, you happy now? :-D