Yesterday I was doing my math (as always) and listening to First Among Sequels by Jasper Fforde (highly recommended!!). I’ve read the book before, which is why it is OK that I listen to it on CD while I do other things. But as I wasn’t really paying attention to the book, I was startled into attention when a riddle was mentioned. As I said, I have read the book, and therefore have also read the riddle, but since I like to speed read my way through the first time, and have not went back and done a reread of the book yet, I hadn’t really worked out the riddle. Well, actually in the book they answer the riddle, but you still have to think about the logic of it to understand it. So I’ll propose the riddle to you, dear readers, and see if any of you are smarter than me and know the answer without having read the book.

The Riddle

There are 2 doors, and in front of each door is a guard. One guard always tells the truth, while one guard always lies. One door leads to JK Rowling and a treasure trove of Harry Potter stuff for life while the other door leads to life serving Lord Voldemort. You do not know which door is which, and you do not know which guard is which. You can only ask one question, and to only one guard, and then you have to choose. What question do you decide to ask? And then which door do you pick? Why?

Please respond in the comments if you will. If there are no answers, or no correct answers, by Monday, I will post the answer!

Mysteriously yours,

lindsayF

P.S. Every time I turn on the TV, and it’s a news program, and they’re talking about the flooding in the Midwest, they always report from an Iowa town. I think that’s funny- that they only are reporting from Iowa…not that the Midwest is flooded, and is going to get even more flooded.

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I think I know the answer, but I am nervous to guess….

Hmm. I think I would ask “What is 1 + 1?” That way, since I already know the answer, I would be able to tell if the guard were lying. Although if the guard were a troll or something, it may not actually know the answer, and then I’m in trouble. Hmm.

P.S. Did the riddle in the book actually include a Harry Potter scenario, or did you just create that to make it even more exciting? Either way, I’m going to be adding this book to my to-read list!

Asking “What is 1+1 tells you whether the guard is lying or not, but you have used your question and have no information about the doors. The question you would ask should be: “What door would the other guard tell me leads to safety?” Consider if Door 1 is the Harry Potter trove and Door 2 leads to Voldemort. If you pose the question to the lying guard he will tell you Door 2, because this is the opposite of what his counterpart would tell you is safe. If the question was posed to the truth teller, he would also say Door 2, because he tells the truth that the liar would mislead you. Either way, the answer to the question is the door you do not want (Unless you’re an aspiring Death Eater).

To Jill: No, it doesn’t really include a HP scenario like I described, but HP is mentioned in the book…it’s actually pretty funny how he’s mentioned.

To Darryl: Congratulations! Have you read the book? Or are you just super smart!?

I’ve never read the book, (but it seems interesting). I have heard the puzzle before, but had to rehash the answer. I’m not super smart by any means, but I do like to think I have a good grip with logic-based problems. I loved the application you put into this riddle. You mentioned flooding in Iowa, where here in Kansas we’re just being blown away with the winds (though Iowa got some of that too). I look forward to the next riddle.

I’d ask one of them: “Would the guard of the other door say that you’re the liar?”

1. If the guard answers NO, you know he is the LIAR.

For him the guard of the other door is the truth teller.

The truth teller would say yes the liar is the liar.

The liar of course would have to lie about what the truth teller would say.

2. If the guard answers YES, you know he’s the TRUTH TELLER.

For him the guard of the other door is the liar.

The liar would say yes the truth teller is liar.

The truth teller of course would have to tell the truth about what the liar would say.

Oops, I missed the second half of the question. Switch out “that you’re the liar” for “that you’ve got JK behind your door.” So the question in full would be: “Would the guard of the other door say that you’ve got JK behind your door?” If the guard I chose said “NO,” I’d pick his door. If the guard I chose said “YES,” I’d chose the other door. Here’s why:

1. If the guard answers NO, you know he’s got JK behind his door because…

If he’s the truth teller, he’ll tell the truth about the liar lying and saying no, JK is not behind his door.

If he’s the liar, the truth teller’s answer would be “yes,” so of course the liar has to say “no” instead.

2. If the guard answers YES, you know he doesn’t have JK behind his door because…

If he’s the truth teller, he’ll tell the truth about the liar lying and saying yes, JK is behind his door.

If he’s the liar, the truth teller’s answer would be “no,” so of course the liar has to say “yes” instead.