Originally Posted by Toodles

Two functions, x^2 and e^x are walking down the street. Off in the distance they see a differential operator walking towards them. e^x turns to x^2 and tells him: "You better get out of here! If that operator differentiates you a few times you'll disappear, but he can differentiate me all he wants and I'll stay the same."

So x^2 runs off and e^x goes to greet the differential operator. "Hi, I'm e^x" he says. "What's your name?"

The operator looks at him with a demonic grin and says: "d/dy".