Pandolfini's Puzzler #10 - E for Excellent!

“You’re doing very well,” Professor Pando said to Lucian. “Come grading time, I may even give you an E for excellent!”
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“But first you must “play detective.” Consider the following position, with the pieces outlining a capital E, which is the letter in question. Oh, by the way, Black has just made a move.”
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White to move
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Question: “How can White checkmate at once, in one move?” 
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“Of course,” said Lucian, “Black must have made a move to get here. Wait. I know what you’re getting at. I have the answer, and I don’t have to violate any rules. I even know Black’s last move.” 
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What rule and move made Lucian laugh?
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Answer below - Try to solve ProfessorPando's Puzzle first!
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ANSWER #10
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Lucian deduced that Black had just played 1…b7-b5. That allows White to counter with 2. c5xb6 mate! – using the en passant rule.
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Take note
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When focused on a strange looking problem, a good question to ask is “How do I think this position came about?” In other words, you should “play detective,” trying to think backward for a bit. That is, trying to think retroactively.


RELATED STUDY MATERIAL

  • Watch PinkHamster's crazy video where you must use en passant to checkmate;
  • Watch his other video on the history and specifics of the rule;
  • Don't crash and burn - GraceLady tells you exactly when to use the special move.