Scooping the loop snooper: An elementary proof of the undecidability of the halting problem.
-- Geoffrey K. Pullum, Mathematics Magazine 73.4 (October 2000), 319-320.

No program can say what another will do.
Now, I won’t just assert that, I’ll prove it to you:
I will prove that although you might work til you drop,
you can’t predict whether a program will stop.