Let’s continue to look at the same idea from a defensive standpoint. Here White sees that Black’s pawn on e5 is defended once (by the pawn on d6) and attacked twice (by the pawn on f4 and the knight on f3). So he is tempted to play 1. f4xe5; then if 1. …d6xe5, 2. Nxe5. But please don’t play an exchange like this until you have imagined what the board would look like afterwards—including what lines would then be open, and with what tactical opportunities then available to you or your opponent. One warning sign is that at the end of the sequence White’s knight on e5, while not attacked, would be loose. Another is that Black’s bishop already attacks f2, adjacent to White’s king. Nothing except the king protects that square (a common state of affairs in the opening; this is why f2 and f7 often are considered defensive weak spots early in a game); so if Black’s queen were able to attack f2 as well, White suddenly would be confronted with a mating threat. The threat could be averted with various moves, but they would take time, and meanwhile Black would be able to take any loose piece his queen also attacked. None of this looks like an immediate worry in the position diagrammed here, but after the exchange of pawns and the recapture Nxe5 by White, the stage would be set for Qd4, threatening mate at f2 and attacking, and winning, the then-loose knight on e5.