It may no longer be at its best (or, in truth, particularly near to its best), but XKCD has been good to us over the years. Last year Vomiting Mike was kind enough to buy me one of their t-shirts as a birthday present (well, actually as three combined birthday presents, because the man is an idiot) based on this particular strip:
Much as I love the shirt, though, it can lead to complications whenever I wear it around the department.
Big Dave: What the Hell is that on your t-shirt?
SS: It's XKCD merchandise. I think the idea behind it is-
BD: Bollocks to the idea behind it, equation 4 is bullshit and chips.
BD: Look at it, Squid; you're multiplying the identity matrix by heart!
BD: So that must equal heart. Not question mark; that's just retarded.
SS: You're assuming a compatible matrix.
SS: Well, what if heart has a non-compatible dimension. 4x5, say.
BD: Then the expression itself is meaningless.
SS: The question mark represents meaningless, you fool, it's a maudlin statement on how
love can't be-
BD: And what's that supposed to be, a probability integral? This is the least mathematically viable piece of clothing I've ever seen.
SS: Christ, Dave, you need one of these t-shirts more than I do.
I love my job.