"eye" motion sensor Answered

Hi guys + gals, i got burning question in my back. Does anybody have any idea how to make a really simple motion sensor? ex: I've been wanting to make something,kinda like an simple installation. It's gonna be a small fishes, so everytime somebody passed my fishes, the eyes of the fish gonna follow the movement of that person. Any help would be deeply appreciated. thank you before hand for the ideas and suggestions.