Lesson
White to move. Main line: 1. e4 e5 2. Nf3 f6 3. Nxe5 fxe5 4. Qh5+ Ke7 5. Qxe5+ Kf7 6. Bc4+
After 3.Nxe5! fxe5 4.Qh5+ the king cannot hide: 4...Ke7 walks into 5.Qxe5+ regaining the piece with check, and 6.Bc4+ brings the last attacker in with the king stranded in the center. (4...g6 5.Qxe5+ picks up the h8-rook instead.)
The point of 2...f6 was to guard e5 without moving a piece — but the f-pawn opens the e8–h5 diagonal and strips the king of the f7 shelter. Defend e5 with a piece (...Nc6, ...d6) and none of this exists.