1.Bxh7+ looks like a free pawn:

1.Bxh7+ Nxh7?
2.Qxf7+ Kh8
3.Qh5! and black will have to give up the queen with Qg6 just to prevent mate. So, at move 1, black has to play

1...............Kh8
2.Bf5 and white continues the attack after the exchange of the lighsquared bishops.