The two-knight surprise

Here's the fact that surprises everyone: king and TWO knights cannot force mate against a lone king. You can give check, you can chase, but every forcing attempt either lets the king escape or produces stalemate — a draw.

(If the defender also has a pawn, it can sometimes become a win, because the pawn gives the king a move and avoids the stalemate. But bare king versus two knights is a draw with best play.)