Пратчетт это наше всё! Вот штучка специально для выпускников матшкол и породнённых с ними лиц™.
"You know," said Ponder, as the coach jolted along a canyon, "this reminds me of that famous logical puzzle."
"What logical puzzle?" said the Archchancellor.
"Well," said Ponder, gratified at the attention, "it appears that there was this man, right, who had to choose between going through two doors, apparently, and the guard on one door always told the truth and the guard on the other door always told a lie, and the thing was, behind one door was certain death, and behind the other door was freedom, and he didn't know which guard was which, and he could only ask them one question and so: what did he ask?"
"Hang on," said Casanunda, "I think I've worked it out. One question, right?"
"Yes," said Ponder, relieved.
"And he can ask either guard?"
"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 color 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.