I never understood the part where Sonny is handed a package with Luca Brasi’s bullet-proof vest wrapped around a fish and asks, “What’s this”?
Clemenza explains that it is a Sicilian symbol that Luca sleeps with the fishes.
How could Sonny, who grew up in the home of a mafia Don, as well as serving as his father’s right hand man, not have known of such a symbol?
Because the audience wouldn’t know and it would be explained. So, Sonny had to play dumb. :)