I like this one from Discworld:

“Hang on,” said Casanunda, “I think I’ve worked it out. One question, right?”

“Yes,” said Ponder, relieved.

“And he can ask either guard?”

“Yes.”

“Oh, right. Well, in that case he goes up to the smallest guard and says, Tell me which is the door to freedom if you don’t want to see the colour of your kidneys and incidentally I’m walking through it behind you, so if you’re trying for the Mr. Clever Award just remember who’s going through it first.'”

“No, no, no!”

“Sounds logical to me,” said Ridcully “Very good thinking.”

“But you haven’t got a weapon!”

“Yes I have. I wrested it from the guard while he was considering the question,” said Casanunda.

“Clever,” said Ridcully. “Now that, Mr. Stibbons, is logical thought".