So it might be that, if you went to a French restaurant and you looked through it and you found they only had margarine and they didn't have butter, you could say, "Ah, I see what's wrong with them.
Come entrare in un ristorante francese, guardarsi intorno e accorgersi che usano solo margarina e niente burro, al che potreste dire "Ah, ecco cos'è che non va.