Sheldon: She befriends them and then lies in wait until they reveal a marketable idea, which she steals and sells to the highest bidder.
Leonard: That's ridiculous.
Sheldon: Oh, is it? Well, let's see you come up with an explanation as to why this woman hangs out with us all the time.
Penny: Oh, great. You know what? I've already mooched dinner off you guys. I don't need to listen to this.
Howard: There's your answer: free food.