Q: How many light bulbs does it take to change a light bulb?
A: One, if it knows its own Goedel number.

Note: I thought this was something to do with the maths/logic theories of Kurt Goedel, about it being impossible to prove things, and finally a more complete explanation arrived in my mailbox : – A Goedel Number is one of several ways to encode a Turing Machine, the classical abstraction of a computer, or for that matter of any algorithm. The idea (as best I see it) is that if the machine knows its own Goedel Number it can simulate itself… It does come from the mathematician Goedel – partly because he used TMs in his famous theorem, I believe.)