Recalculate

I teach young children that when they calculate a sequence of moves, they should then recheck after the first move is played. My failure in this tactics problem on Tactic Trainer this morning illustrates this point. Although I found a checkmate, I failed the problem because there was a shorter checkmate.

Black to move

Black is at Bottom

I found 1...Nf4 and White's obligatory 2.g5. I then played without further calculation 2...Ng2+ with the idea 3.Kh3 fxg5 4.b7 Nf4#.

I missed 2...h6! with mate forced on the next move.

Perhaps it is not fair that my training rating went down nearly six points for a checkmate in four when there was a checkmate in three, but those are the rules of the training app. I accept these rules.