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Alex, Boris, and Jasper are at a bar celebrating Cinco De Mayo. Alex announces he can read Jasper’s mind and goes out for some fresh air. Boris asks Jasper to select something in the room. Jasper points at a lime. When Alex returns, Boris asks, “Is it that napkin?” Alex says, “No.” Boris asks, “Is it the night sky?” Alex says, “No.” Boris asks, “Is it that lime?” Alex says, “Yes.”

Alex leaves the room again, and Jasper points at an ashtray. When Alex returns, Boris asks, “Is it that spoon?” Alex says, “No.” Boris asks, “Is it that cigarette?” Alex says, “No.” Boris asks, “Is it your shadow?” Alex says, “No.” Boris asks, “Is it that ashtray?” Alex says “Yes.”

This is repeated again and Jasper points at a straw. When Alex returns, Boris asks, “Is it that cigar?” Alex says, “No.” Boris asks, “Is it that fork?” Alex replies, “No.” Boris asks, “Is it that rose?” Alex says, “No.” Boris asks, “Is it that crow?” Alex says, “No.” Boris asks, “Is it that straw?” Alex says, “Yes.”

Jasper is very much amazed that Alex was able to guess it right thrice in a row. How do you think he did it?

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closed as too broad by xnor, Len, A E, dmg, mdc32 Mar 5 '15 at 15:35

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ A similar puzzle. I wouldn't call it a dupe though. $\endgroup$ – Rand al'Thor Mar 4 '15 at 11:32
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    $\begingroup$ Are you sure Boris asked "Is it that crow?" and not, say, "Is it the crow?" $\endgroup$ – Joel Reyes Noche Mar 4 '15 at 14:49
  • $\begingroup$ There are just too many ways that this could be done for us to answer, "How do you think he did it?". I think this is too broad. $\endgroup$ – xnor Mar 4 '15 at 22:39
  • $\begingroup$ I agree with @xnor, the Black Magic idea fits the original question, but the comments say that it is not. $\endgroup$ – Shadow Z. Mar 5 '15 at 1:57
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The trick is

Boris always asks "Is it [something not in the bar]?" just before coming to the thing Jasper actually pointed to.

So Alex knows

to say "no" up until the first object mentioned after one that's not in the bar.


Another example that works (supplied by the OP):

Alex leaves the room again, and Jasper points at a wineglass. When Alex returns, Boris asks, “Is it that knife?” Alex says, “No.” Boris asks, “Is it that bottle?” Alex says, “No.” Boris asks, "Is it that chair?" Alex says, "No." Boris asks, “Is it the bed?” Alex says, “No.” Boris asks, “Is it that wineglass?” Alex says “Yes.”

Another one (assuming I'm right):

Alex leaves the bar again, and Jasper points at his mobile phone. When Alex returns, Boris asks, “Is it that beer glass?” Alex says, “No.” Boris asks, “Is it that table?” Alex says, “No.” Boris asks, “Is it my mobile phone?” (having left it at home). Alex says, “No.” Boris asks, “Is it Jasper's mobile phone?” Alex says “Yes.”

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  • 1
    $\begingroup$ You may have to edit your solution. Consider this: Alex leaves the room again, and Jasper points at a wineglass. When Alex returns, Boris asks, “Is it that knife?” Alex says, “No.” Boris asks, “Is it that bottle?” Alex says, “No.” Boris asks, "Is it that chair?" Alex says, "No." Boris asks, “Is it the bed?” Alex says, “No.” Boris asks, “Is it that wineglass?” Alex says “Yes.” $\endgroup$ – Donbhupi Mar 4 '15 at 11:51
  • $\begingroup$ "Is it a black Mercedes?" is obviously No as "a" refers to something that is not around. $\endgroup$ – Spikatrix Mar 4 '15 at 14:03
  • $\begingroup$ In elementary school, we called this game "Black Magic" $\endgroup$ – Corey Ogburn Mar 4 '15 at 16:22
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    $\begingroup$ @kaine , in his comment here, did get it exactly how it was intended to be. $\endgroup$ – Donbhupi Mar 5 '15 at 9:38
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    $\begingroup$ Why wouldn't a shadow be located in a bar? There should be light sources to the point of vision, and any amount of light will create a shadow (unless it is lit up from all sides/walls/areas, but that seems like a very poor design for a bar). $\endgroup$ – Shadow Z. Mar 23 '15 at 17:31
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Another potential possible answer (though likely a coincidence) is that there is a preselected response pattern for Alex. The question as is follows this pattern:

  • Game 1: No, No, Yes
  • Game 2: No, No, No, Yes
  • Game 3: No, No, No, No, Yes
  • Potential Game 4: No, No, No, No, No, Yes

As is, Boris is in complete control of the situation and there are several possible valid strategies.

Addition

As this apparently is more popular than I thought it would be, I thought I would summarize the solutions I know. There are 5 classes of solutions I am aware of. All of them are predetermined strategies between Alex and Boris.

  1. Alex's answers are predetermined. As Boris is in complete control of what questions are asked when, he can simply ask the question which warrents a positive response when he already knows Alex is about to give a positive response. This is the answer described above. My current favorite version which fits with the example situation is using every other number in $\pi$ to indicate the question Alex should answer yes to. This would be "3,4,5,2,5,5,etc". This would be very difficult to detect compared to other strategies. If they run out of digits they have memorized, use the next strategy to indicate that they need to restart the pattern.
  2. Boris signals to Alex which answer is correct with non-verbal communication. Nothing in the question indicates this is the case but it is so easy to do and so flexible that Jasper could likely not tell. If, for instance, Boris puts his thumb inside his hand when pointing to the correct object and outside of it when pointing to the wrong one. If this is what Jasper is looking for, it is the easiest to detect. If, however, he is watching Alex and Alex follows the hand to the object with his eyes, Jasper probably won't ever figure it out.
  3. Boris communicates to Alex by verbal means. This is probably the expected answer. By cleverly choosing the wording of the correct question or a previous one, one could feasibly communicate which one is correct without relying on a memorized pattern. This is in other answers already. It is worth noting that if you use a previous question to communicate the correct information, there will never be a positive response (Yes) on the first question which I would see as suspicious.
  4. Truely cheat. If all else fails, setup a bug so Alex can hear Jasper's request.
  5. Combination platter. While any and all of these strategies could explain the example situation, if one wants to reliably and repeatedly do this in front of several people the correct option is to use a combination of these three strategies. This is throw anyone off if they start to figure it out. Actually figuring it out would be impossibly difficult.

For a type 5 example, Boris could use the word "that" in every example except the one before the correct one but if Boris refers to something not present like a crow, the next prime number question is correct. These are both overridden by Boris breathing through his mouth when Alex comes back in. If he does that, the audience chose all the questions so Alex should say "Yes" on the question during which Boris blinks. If not communication is attempted at all, he should answer yes on question $(n^2 \mod 5)+10$.

Intentionally, this gets ridiculously complicated but that is how many kinds of magic involving audience participation can repeatedly work and amaze. You must repeatedly remain so flexible that the audience's demands can't keep up.

The point of this breakdown is that there are several ways this can be done and the best way is to remain flexible. Also, cheating makes magic easier.

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    $\begingroup$ How does this work? Boris would have to guess the correct object on a certain guess, because Jasper gives Alex the actual object. $\endgroup$ – mdc32 Mar 4 '15 at 17:38
  • $\begingroup$ @mdc32 on round x, alex should just say yes after question (x+2). Boris knows this and intentionally asks the question which should be answered yes then and only then. $\endgroup$ – kaine Mar 4 '15 at 17:42
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    $\begingroup$ Jasper is the one only who is not in on it. Boris asks Alex the questions and choses what to ask. Alex answers yes or no. Jasper only influences what is the correct question to ask when Alex is going to answer "Yes" either way. $\endgroup$ – kaine Mar 4 '15 at 18:19
  • $\begingroup$ I hope this is a coincidence! Otherwise the puzzle isn't as interesting as it looks - the game couldn't reasonably be continued forever. Also the OP's comment on my answer, giving a new example, suggests your answer isn't what was intended. $\endgroup$ – Rand al'Thor Mar 4 '15 at 21:58
  • $\begingroup$ @rand al'thor. The game could easily continue with the correct preset rotation. Instead of 3, 4, 5, 6, 7... they could pick 3,4,5,3,4,5,3,4,5... or 3,4,5,3,1,4,1,5,9,2,6,5,3... or any pattern they like. It currently could just be every other value of pi. This would ( long term) be less obvious as first question correct answers are possible. $\endgroup$ – kaine Mar 4 '15 at 22:02
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I belive it is based off a "magic trick" called Black Magic. To proove that they are communicating telepathicly, they follow a rule:

The item listed after the black item is the item listed.

(Hence the name, Black Magic).

Each item before the last one fits the criteria for Black Magic to work.

In the first one:

Jasper points at a lime. When Alex returns, Boris asks, “Is it that napkin?” Alex says, “No.” Boris asks, “Is it the night sky?” Alex says, “No.” Boris asks, “Is it that lime?” Alex says, “Yes.”

The night sky is usually a black object (without stars and stuff), so it fits the criteria for Black Magic.

In the second one:

Alex leaves the room again, and Jasper points at an ashtray. When Alex returns, Boris asks, “Is it that spoon?” Alex says, “No.” Boris asks, “Is it that cigarette?” Alex says, “No.” Boris asks, “Is it your shadow?” Alex says, “No.” Boris asks, “Is it that ashtray?” Alex says “Yes.”

A persons shadow is colored black, so it also fits the criteria.

In the third one:

This is repeated again and Jasper points at a straw. When Alex returns, Boris asks, “Is it that cigar?” Alex says, “No.” Boris asks, “Is it that fork?” Alex replies, “No.” Boris asks, “Is it that rose?” Alex says, “No.” Boris asks, “Is it that crow?” Alex says, “No.” Boris asks, “Is it that straw?” Alex says, “Yes.”

Crows are known for being black birds, so it too falls under the same criteria.

Alex just has to make sure he notices the item that is black that Boris states, and he will be able to "telepathicly" figure out which item is the correct item.

There are other ways to do it, like the nth "is it __ ," but the Black Magic way allows it to be much more flexible, and could either be fast or short. Just not on the first one, while the nth "is it __ " way could apply to the first one (if said so before hand), but that could cause it to be dull.

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  • $\begingroup$ Nice guess (actually the same as rand al'thor's original answer), but the OP's example in a comment is not consistent with it (knife, bottle, chair, bed, wineglass) (unless the bed is black, but this was not stated). $\endgroup$ – Joel Reyes Noche Mar 5 '15 at 1:49
  • $\begingroup$ That was my only guess, I was just answering based off of the original question without any of the comments in other answers. $\endgroup$ – Shadow Z. Mar 5 '15 at 1:55
  • $\begingroup$ +1. This is what first came to my mind also, and consistent with the question. Perhaps OP doesn't understand (or misunderstood) the answer himself that he can't give consistent answer. I believe this should be the answer. But it is also possible that OP is thinking a variation of this famous game :D $\endgroup$ – justhalf Mar 5 '15 at 6:12
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Basically, there has to be some (hidden) way for Boris to communicate what the secret object is to Alex, probably by either indicating when the question to which he should answer "yes" is coming up, or by signalling how many times to say "no" before answering "yes".

My guess is that Alex knows to answer "yes" to the next question when

Boris asks about something that shares the same first letter as an earlier guess.

The guesses that precede the correct answer in the examples are "night sky," "shadow," and "crow". In each example,

These guesses are preceded by "napkin," "spoon," and "cigar," respectively.

This also fits with the OP's example in a comment, as

The second-to-last guess, "bed," is preceded by a guess of "bottle".

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  • $\begingroup$ Nice guess, but the OP's example in a comment is not consistent with it (knife, bottle, chair, bed, wineglass). $\endgroup$ – Joel Reyes Noche Mar 5 '15 at 1:48
  • $\begingroup$ @JoelReyesNoche It is now. $\endgroup$ – KSmarts Mar 5 '15 at 14:46
  • $\begingroup$ This is very nice! $\endgroup$ – Joel Reyes Noche Mar 6 '15 at 0:48
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My guess is that the questions start with "Is it that ..." and once the question has the third word different ("Is it the ...," "Is it your ..."), then the next item is the one pointed to by Jasper. The only problem with my guess is the question "Is it that crow?" Are you sure Boris didn't ask, say, "Is it the crow?"

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