From what I've read, a rat is about as smart as the average dog. I read one article about a research project that found that rats "giggle" when they're tickled. Apparently, it's a high-pitched sound above the hearing range of most people. It was an interesting article.
I used to know people who bred rats to sell to pet stores. They were really into breeding for certain colors and color patterns. They even had shows, kinda like dog shows. I learned to like domestic rats. They aren't as prone to bite as mice and hamsters and other small rodents. That doesn't mean that I care to have rats ... or any other animal, for that matter ... move in and take over my home.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams