Our little visitor
-- Lettuce comes from plastic bags, which make a distinctive noise when they're opened.
-- If he started squealing at the sound of a plastic bag -- any plastic bag -- being opened, eventually one of us would cave him and give him some lettuce. Yes, he trained us well.
The guinea pig also had the entertaining habit of jumping up onto the top of his cardboard "house", then jumping off and running inside, only run back out a moment later. I once watched him do this five times in a row.
Sadly, he is back at school, and things are a little less amusing around the house. On the plus side, we have our dining room table back; if it weren't for the fact we have no place else to put a guinea pig, I suspect we would have one of our own by now.