This question has come up a lot recently in both the rabbit and guinea pig forums. So here is the long and full blown answer.
Language Guinea pigs use both body and vocal language. Rabbits cannot understand the vocal side of things, therefore the rabbit finds this frustrating...
They look at you with sad but eager eyes through their cages. You may have seen them in the mall, online or in your local pet store. Maybe you have even visited the shelters. There they are. Their little faces look at you with hope that you might be the one to take them home.