Everyone Made a Statement. Exactly One of Them Is Lying
A short elimination puzzle with a single consistent answer, checked by brute force against every possible culprit before publication.
This is the smallest interesting logic puzzle there is. A handful of people, one statement each, and a single guarantee: exactly one of those statements is false.
That guarantee is doing all the work. Without it there is nothing to go on. With it, the puzzle becomes a counting exercise.
The method, in one paragraph
Take each person in turn and assume they are the culprit. Under that assumption, go through every statement and mark it true or false. Count the false ones. If the count is anything other than one, that person cannot be the culprit, because it would contradict the guarantee. Do that for everyone, and exactly one name will be left standing.
That is genuinely all there is to it. It takes about two minutes with a pen, and about thirty seconds once you have done a few.
Why people find it hard anyway
Because the natural instinct is to look for the person who seems most suspicious, which is a completely different mental operation. Denials feel guiltier than accusations. Somebody who says nothing about themselves feels evasive. None of that is relevant here — the only thing that matters is the arithmetic of how many statements come out false.
It is a useful reminder of how badly intuition performs on this kind of problem, and it is exactly why the puzzle is satisfying when it lands.
The guarantee behind the guarantee
This puzzle was generated and then solved by testing every suspect in exactly the way described above. It was only accepted for publication because precisely one name produced exactly one false statement. If two names had worked, or none, it would have been thrown away and regenerated.
Work it out, then open the reveal. It names the culprit and the person telling the lie, which are not always the same person.
Medium Someone finished the last of the milk and put the empty bottle back in the fridge. All 4 of them were asked, and exactly one of them is lying. Who did it?
Reveal who was lying
Answer Dev
It was Dev, and Gus is the one telling the lie. Test each name in turn: assume that person is responsible, then count how many of the 4 statements come out false. Dev is the only name that leaves exactly one false statement — every other name leaves either none or more than one.
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