Source: by Karl Fabel & was reprinted in Martin Gardner's New Mathematical Diversions from Scientific American (pub. Simon & Schuster), ch.19, p.223
I suspect the trick is, as with so many chess puzzles, that:
we are looking at the board the wrong way.
In this case, there are several possible moves:
Actually, I think there's no move that can force mate, if this is the case, because now the pawns are 'blocking' the wrong squares.