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Not exactly a traditional chess puzzle, and I won't offer much of an explanation for fear of giving away the answer.

White must mate in 0 moves, meaning he must deliver a check mate without touching any of his pieces. How can he do that?

enter image description here

Courtesy of: Chess Fruits, 1884

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    $\begingroup$ Let Chewbacca play White. $\endgroup$
    – Cœur
    Oct 22, 2018 at 5:58
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    $\begingroup$ I knew it as "mate in ½ move". $\endgroup$
    – angus
    Oct 23, 2018 at 10:50
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    $\begingroup$ @angus $\frac14$ ? i mean $\frac12$ is like half-move, but i know what you mean. very clever as well. $\endgroup$
    – BCLC
    Aug 20, 2021 at 13:24

5 Answers 5

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White must

Remove the black pawn on c5 that she has just taken en passant.

Indeed

One move ago, wPc6 was on b5 and bPc5 was on c7. Black played c7-c5 and White took b5xc6 e.p. It just remains to remove the black pawn.

Note that

e7-e5 for Black followed by f5xe6 e.p. for White is not an option, because then you cannot explain how black pawns arrived on g6 and h4.

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    $\begingroup$ If this is correct, then technically the image of the chess board is not in a legal state. $\endgroup$
    – forest
    Oct 19, 2018 at 10:43
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    $\begingroup$ @forest It's in a legal state, but white's next action must be as specified to remain legal. After all, it's possible that this state was achieved because a fire alarm sounded right before that exact moment. $\endgroup$
    – Michael
    Oct 19, 2018 at 16:03
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    $\begingroup$ @greenturtle3141 : unless we know that a 'mate in 0' is possible, we cannot prove that this is how the position was reached. $\endgroup$
    – Evargalo
    Oct 19, 2018 at 21:05
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    $\begingroup$ @Evargalo: All such chess puzzles are critically defective because chess puzzles exist that originate from impossible positions. $\endgroup$
    – Joshua
    Oct 21, 2018 at 19:47
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    $\begingroup$ It becomes a question of definition. Is it a legal state to have the king not on the board? Physically you remove the king from the board in the act of castling, only to redeposit it on the board somewhere else. I don't see any use (beyond clever puzzles perhaps) for calling physical configurations of the chess board "states" if those physical configurations don't correspond to a position which is reached after a legal move. $\endgroup$
    – John Coleman
    Oct 23, 2018 at 11:51
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I see Evargalo has found the right answer, but the first thing that occurred to me was

rotate the board 180 degrees. This will cause White's pawns which are currently on c6 and e6 to be attacking Black's king, and Black has no way to get out of check.

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    $\begingroup$ I thought someone might try that, that's why I made sure to include the letters/numbers in the picture :) $\endgroup$
    – JGibbers
    Oct 18, 2018 at 16:44
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    $\begingroup$ Well, obviously the part with numbers and letters wouldn't be included in the rotation. $\endgroup$
    – Acccumulation
    Oct 18, 2018 at 17:03
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    $\begingroup$ That's double-check with two pawns, which is not possible (and the post is tagged retrograde-analysis so it's assumed there was a previous move) $\endgroup$
    – BlueRaja - Danny Pflughoeft
    Oct 18, 2018 at 21:30
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    $\begingroup$ I like this answer much more than the correct one. $\endgroup$
    – Surb
    Oct 19, 2018 at 8:44
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    $\begingroup$ How does rotating the board change anything? $\endgroup$
    – Fabian Röling
    Oct 20, 2018 at 23:56
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Not exactly a checkmate, but there is an option with a similar effect that can be performed in zero moves:

Resign.

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    $\begingroup$ Welcome to Puzzling SE. Make sure when you answer questions you fully explain your thoughts behind it and approach. $\endgroup$
    – gabbo1092
    Oct 19, 2018 at 16:52
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    $\begingroup$ @gabbo1092 Thanks. I'd be happy to elaborate, but I'm not sure what else there is to say. I suppose I could explicitly state that a resignation, like a checkmate, ends the game with a loss (and does so in zero moves), but I assumed anyone reading chess problems knows that already. Is there anything in particular that I should explain more clearly? $\endgroup$
    – Ray
    Oct 19, 2018 at 17:00
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    $\begingroup$ Sorry entirely my bad somehow I had misread the answer and misunderstood. hope you enjoy your time her on PSE $\endgroup$
    – gabbo1092
    Oct 19, 2018 at 17:04
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    $\begingroup$ I wouldn't call winning and losing "similar effects" $\endgroup$
    – buzjwa
    Oct 21, 2018 at 6:33
  • $\begingroup$ @buzjwa Fortunately, both winning and losing happen, just to the opposite players they usually happen to in chess puzzles. It's definitely a stretch, but there doesn't exist a traditional solution to this one. $\endgroup$
    – Ray
    Oct 21, 2018 at 14:19
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I don't think this is within the rules of the game, but rotating the board 180 degrees would mate the black king. Obviously, the board would be setup incorrectly at that point, but doing anything in 0 moves in chess is illegal, so I would say this is the best answer.

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    $\begingroup$ Welcome to Puzzling.SE, @lustig! Innovative twist (no pun intended)! $\endgroup$
    – SteveV
    Oct 21, 2018 at 19:37
  • $\begingroup$ Thank you! Just realized the answer was already posted but didn't read through them all. Apologies, but will leave unless deleted by mods. $\endgroup$
    – technoplato
    Oct 21, 2018 at 20:13
  • $\begingroup$ Turns out my answer was NOT correct and had been debunked by the puzzle "host"; and yet here I was grumpily checking why my answer didn't have more votes. How silly I was ... puzzling.stackexchange.com/questions/74086/mate-in-0-moves/… $\endgroup$
    – technoplato
    Apr 2, 2022 at 8:28
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[...] check mate without touching any of his pieces?

So therefore white should be able to:

Move the black king to d8.

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    $\begingroup$ On f7 the bK wouldn't be mated (e.g. it can go to f6) rather move it to d8 ? $\endgroup$
    – Evargalo
    Oct 22, 2018 at 12:03
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    $\begingroup$ Correct, missed that mate! (pun intended) $\endgroup$
    – Viktor Mellgren
    Oct 22, 2018 at 12:16

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