I really liked the latest comic on Order of the Stick. I suppose I found it funny because when I was very young I had classes in non-linear thinking, or as I prefer to think of it, classes in how to be a smart-ass. When I was about 13 or so I came up with a fairly similar solution to the same puzzle in a D&D game which annoyed my DM to no end (I simply asked one of the two characters their gender, since the DM had already specified that they both were men).

Incidentally the correct question is ‘if I ask the other person if you lie what will they say?’ If you ask the truth-teller then he will answer truthfully that the other person would claim that the, he truth-teller, is the liar. On the other hand if you ask the liar what the truth-teller would say then they will lie, and since the truth-teller would say that the liar is the liar then the liar has to claim that the truth-teller would claim that the liar tells the truth.