Knights aren't the only forking piece, pawns fork too, and they're the cheapest piece on the board to lose. A pawn advancing one square can suddenly attack two pieces at once diagonally, and since a pawn is worth so little, the trade is almost always lopsided in your favor.
This is why advanced players are careful about where they let their knights and bishops sit. A piece parked on a square a pawn can jump in front of, hitting it and something else at once, is an accident waiting to happen.
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White's pawn on c4 can step forward to a square that attacks both knights at once.
Which pawn move forks both knights?
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