How is it possible for a television commerical to be "popular"?

I was reading about the new tv series Cavemen and I was curious as to how the Neilsen ratings could track the viewership of a commercial. For one, commericials are very short and two, watching commericials is inevitable. For example, if you are watching American Idol and it goes to a commericial, you don't really choose to watch it or not. It's just your luck that you're still sitting on the couch. So how then is it possible to know that people really like a commericial?