My son got a guinea pig for his birthday back in May. It is a very smart guinea pig because it now has my wife and I trained to give it treats. Whenever she or I walk past the bed room door, this thing starts whistling like crazy until we give it a carrot or lettuce or something. It never does it with the kids, just with her and I. It must recognize the heavier footsteps or something.
It also has an interesting way of telling us it doesn't like it's food. We got some pellets that had dried vegetables mixed in. It eats all the pellets, leaving just the dried veggies and then poops in the food dish, apparently to let us know we can get rid of the rest. It never does this when there are pellets in the dish.
I didn't realize that these things were that smart.