Chess in 40 SecondsThe Lasker Trap
Black to move. Promote with check.
Your pawn on f2 is about to promote. Pick the right new piece.
Hint
Promote, but not to a queen.
Answer
fxg1=N+ — fxg1=N+! Promoting to a knight gives check and wins. A queen would let White escape.
The idea
A knight can give check where a queen can't. Sometimes the knight is the best promotion.
This chess trap is one of 30 quick puzzles in the Chess in 40 Seconds trainer: find the move on the board, get a hint if you're stuck, and learn the idea so you never fall for it in your own games.