I can't take credit for this one; it's actually from philntex. I found it pretty funny in the sick, "It's disgusting that I actually understand this joke" sense.
E-to-the-x and a constant are walking down the street. Suddenly, in the distance they see a differential opperator. The constant shrieks out "Oh no, I'll be reduced to zero," and hides in the bushes. Meanwhile, e-to-the-x, is like, "Hey!, I'm e-to-the-x. I'll show that differential opperator not to mess with me." So, e-to-the-x goes up to the differential opperator and says, "Ha, I'm e-to-the-x, your evil powers are useless against me." The differential operator then busts out laughing and says, "Really? I'm d/dy."