In my area, most kids including me grew up with a very popular house-rule: After all the pieces of either of the players are removed besides the king, the opposing player has 18 moves to checkmate him. If he can't do that, the game is a draw.
The rule is popularly called 18-moves.
I have spent some time googling and have not found this rule anywhere.
Does anyone know a source for this rule and why it's so popular?