[broken link! FIXME] Rats can be made to laugh, apparently. They like to be tickled, according to this article.
I had a pet rat, once. I found a rat to be a surprisingly affectionate and comforting pet. I remember that he would "purr" when he was curled up and I was petting him. My rat was named "Fnugus" – which was some strange whim of my adolescent mind. I ran across a picture of my long-ago pet rat, recently, in my collection. It's not a great photo, but it made me nostalgic.
Fnugus RIP – 1979~1982.
Nostalgic for a rat. Rats!