Martin Gardner mentioned the following "chess" problem in his column, Mathematical Games, for Scientific American (he got it from Smullyan, who learned about it from a high school math teacher):
... Assume that a game is played with the new rule that a knight can on a single turn move twice. It is not clear if this more powerful knight move is an advantage to the first or second player. The first player, White, offers to play the game without his queen. How can he be sure to win?
After thinking a bit, I could not make progress, so I look at the solution:
White jumps queen's knight forward twice. Checkmate!
That's all it says. How is this a checkmate? I must be missing something.