My 13-yo son and I invented a new type of chess last night. Starting board: he white, me black, black to move first.

The rules: White to mate within 20 moves. If it takes longer, or if Black is stalemated, or if Black takes any of White's pieces, Black wins.

What we quickly discovered was: if White retains their Queen, the game is an easy win. Without a Queen it's harder, but doable. But without Queen, one Bishop and one Knight, it gets surprisingly tricky. So this set-up:

It's possible (likely even) that somebody else has invented this variant already --- there are a million chess variants out there, I know. And it's possible that there's an easy way for White to play and guarantee a win; but if so we haven't found it yet.