When is a fish not a fish?

So you go to the fishmongers. You want some mackerel, you know it’s good for you. Feeling confident, you ask: ‘Tienes algun caballo?’ The fishmonger laughs, and goes into a shtik, miming the riding of a horse. Doh! You’ve asked for a horse (caballo), not mackerel (caballa). Well, you won’t do that again, will you?