clapping

Why do people clap after a movie finishes. I have been in movies where people just start clapping after the movie ends. I never understood it. Do they think the movie will hear it? Do they think there are the movie makers in the back watching the film. Or are they so used to going to plays and musicals that they can't help themselves. I understand group mentality. So I understand why people clap after other people are clapping. But what about the first person that is clapping? Maybe they are trying to wake up their dates who fell asleep by clapping?