Usually the beginning of a story that people hear a lot. For example, "My girlfriend is upset about her new haircut" or "My dad keeps losing his car keys." And then I just think of different ways the story could end. "My girlfriend is upset about her new haircut. I don't understand why she's crying. I'm the one who has to get a new girlfriend." Then I try it out on stage. I don't do a lot of re-writing. My jokes either work or they don't. The trick is just to write a ton of jokes.