Jack and Joe are fighting over a robot that always saves its owner.

They both claim to have created it, but only one is telling the truth.

Neither would be able to live with the other taking the credit, so Jack offers Joe two identical-looking drinks, saying one is poisoned.

Whichever one Joe drinks, Jack will drink the other.

Joe believes he has a 50% chance of survival, but Jack knows it's less. How?

  • $\begingroup$ The liars tag would fit nicely here I think. $\endgroup$ Jun 17, 2016 at 21:22
  • $\begingroup$ @BrentHackers Notice it doesn't say that one of them always lies. $\endgroup$
    – user88
    Jun 17, 2016 at 21:23
  • $\begingroup$ @JoeZ. Fair point. $\endgroup$ Jun 17, 2016 at 21:24
  • $\begingroup$ Does the robot know who made it? $\endgroup$
    – ash4fun
    Jun 17, 2016 at 21:30
  • $\begingroup$ @rybo111 Given that you've tagged the riddle as "situation", either the question must be answerable from just the information given in the question statement, or you're obliged to answer any closed-ended questions that anyone asks you about the situation. $\endgroup$
    – user88
    Jun 17, 2016 at 21:37

3 Answers 3


Jack has actually poisoned both drinks (saying that one drink is poisoned doesn't mean the other one isn't poisoned as well), but he's the one who created the robot, so he'll live somehow through the robot saving him, by giving him an antidote or some other way.

In this scenario, the probability that Joe survives is actually 0%.


A technicality maybe? But...

You specify 'identical-looking' drinks. Could they smell different or something? I don't expect this would help if jack isn't picking his own drink... But if Joe made a decision already then jack could take a whiff and be 100% sure he'll live.


is the key to the answer that "Neither would be able to live with the other taking the credit" so if jack (the creator) is poisoned the robot at least tries to save him and then Joe would die because he couldn't take credit any more? Otherwise joe dies of poisoning. Joe has a 0% chance to live!


The poison is non-lethal. Jack knows both have a 100% chance to survive drinking BUT the robots reaction or lack of one will reveal the creator and the other man (Joe) will kill himself or die of embarrassment giving him no chance to survive.

  • $\begingroup$ His survival chances pre-selection then? $\endgroup$ Jun 17, 2016 at 21:44
  • $\begingroup$ @rybo111 How's my alternative? $\endgroup$ Jun 17, 2016 at 21:53
  • $\begingroup$ @rybo111 or my other alternative... $\endgroup$ Jun 17, 2016 at 22:06
  • $\begingroup$ OK but how come those wouldn't work? Is the statement about them not being able to live without taking credit litterelal? $\endgroup$ Jun 17, 2016 at 22:26
  • $\begingroup$ Oh. OK. I didn't assume that the survival chance was purely based on choosing and/or drinking the drinks. To clarify, jack has poisoned only one of the two drinks, the poison is lethal, and the result of drinking (as in the poison) is the only thing that has any baring on Joe's survival in the context of the question? $\endgroup$ Jun 18, 2016 at 5:59

First off

The odds are 50% for being the robot's owner, if it's really so closely contested they're both willing to gamble their lives on it... not a lot of room for someone knowingly lying about it. The odds are also 50% for picking the poisoned glass, and that is not dependent on ownership. These are not the same 50% chances - so, the chances work out among four options (owner is or isn't poisoned, not-owner is poisoned or not). On the other hand, three of the four are nonlethal options, a 25% chance of being fatal (both not-owner and picked poison) so that would make sense if it was only about Joe being wrong about his chances, not overestimating. Also only a 25% chance of getting the robot to act (both owner and picked poison), so not terribly useful, either.

So, perhaps we can assume

both drinks are poisoned. But that only brings it back down to 50% odds, owner or not, since the glass picked is now irrelevant. However, Jack goes first - this is given. Perhaps the extra odds against his survival have to do with the potency of the poison? If the poison is very quick, the extra time could nibble away the odds of survival even if an antidote is given. Or it could increase Joe's chance of surviving (as in, he can take an antidote based on what he sees happening to Jack), but increasing Jack's odds don't decrease Joe's odds.

hmm, what if

they're different poisons, and the robot only has the antidote to one? then only the owner will survive, reversing that previous 75% odds to against Joe (must be both owner and pick the right glass to live). On the other hand, there's 25% chance against the owner surviving anyway in this scenario, so maybe that's a bit extreme. On the other hand, the question does say they can't live with the possibility of the other taking the credit, so I think this is going to be my answer. That, or Jack's poison is so unique only Jack's robot will have the antidote - so if Joe is not the owner, he definitely dies (50% chance), if he is the owner, he might die anyway (some extra % chance the robot will not have the correct antidote). Jack certainly lives, since owner or not he has the right antidote.

Final answer!

Both glasses are poisoned, and the robot is openly given "the antidote"...which only counters one of two poisons, in scenario one, or is outright false, if scenario two. The first possibility is with each glass having different poisons. This would lead to 75% odds against either of them surviving, since to live they must be both the owner and have picked the right glass. Or, possibility two is that perhaps they are both poisoned with some incredibly proprietary poison of Jack's manufacture, because if the robot is his, it has the right antidote... but if it belongs to Joe, there is some extra % chance it won't have or be able to get the real antidote in time.


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