Probably not a riddle, but it sure needs logic.
My friend, the chess nerd, called me about a chess puzzle he has seen. He gave me the following position over the phone (set it up on a chess board). Note that it is not real (king vs king is impossible), it is just theory.
White king on a1
Black king on a8
Now, white to move and reach either f8, g8, or h8 BY FORCE. Have fun.
Please try to give variations (with best play from black), as 1 move per post is tedious to read.