jose's post reminds me of one I heard Michael Hutchings tell during an undergraduate calculus lecture:
$e^x$ was walking down the street one day and met a polynomial running in the opposite direction.
"Wait, why are you running?" asked $e^x$. The polynomial said:
"There's a differential operator over there! It could differentiate me and turn me into zero!" And the polynomial continued running in fright.
"Ha ha," $e^x$ said to himself. "I'm $e^x$! Let them differentiate me as many times as they want, it makes no difference to me!" So $e^x$ walked on and reached the differential operator. He confidently introduced himself: "Hi, I'm $e^x$!" The reply:
"Hi, I'm $\partial/\partial y$!"
