Below is my favorite math joke (copied it from http://www.onlinemathlearning.com/math-jokes-calculus.html as I didn't remember the exact wording). It's a bit long for a T-shirt, but a simple comic strip version could be made.

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A constant function and e^x are walking on Broadway. Then suddenly the constant function sees a differential operator approaching and runs away. So e^x follows him and asks why the hurry. "Well, you see, there's this differential operator coming this way, and when we meet, he'll differentiate me and nothing will be left of me...!" "Ah," says ex, "he won't bother ME, I'm e to the x!" and he walks on. Of course he meets the differential operator after a short distance.

e^x: "Hi, I'm e^x"

diff.op.: "Hi, I'm d/dy"