En passant

Also called: en-passant capture

En passant is a special pawn capture. If an opponent's pawn moves two squares forward and lands beside your pawn, your pawn may capture it as if it had moved only one square. It's the only capture where the captured piece isn't standing on the capturing piece's destination square.

When your child meets this: it’s the single most confusing rule for beginners, because nothing else in chess works this way — a pawn disappears without the capturing piece landing on it. Most children first encounter it by accident, either doing it without understanding why it’s legal or getting caught by it and assuming their opponent cheated. It usually takes a coach walking through two or three real examples on the board before it clicks — don’t worry if you need to look it up again yourself before you can explain it back.