Survey your attacks. Black’s rook and bishop attack White’s knight on d5 (which is protected twice, by a pawn and another knight). Black’s queen attacks White’s knight on e3, which is protected by a pawn and by White’s queen. Black’s knight attacks White’s bishop on e2, which is protected by its queen. Can any of the defenders mentioned in those observations be captured or attacked? We saw that White’s knight on d5 is attacked twice and defended twice. If one of its defenders were removed, it could be taken. One of those defenders is the pawn on e4. Black can capture it with f5xe4, which also is a pawn attack on White’s queen. Now what? White might consider Qxe4, replacing his lost pawn with a piece that still protects the d5 knight. But remember that White’s queen had defensive work of its own to do on d3: it was protecting the bishop on e2. Once it leaves for e4, Black has NxB+, winning a piece. Black’s initial pawn move can be considered a capture or an attack on White’s queen; the important point is just to see the train of thought that resulted one way or another in the removal of a valuable White guard.