Negative images
　　Light bulb jokes are an example of satire[讽刺]， and are funny when the audience agrees with the stereotypes used in them； and like most stereotypes， they’re usually negative.
　　How many plumbers does it take to change a light bulb？
　　Three： One to come to your house the day before， when you’re out； one to change the switch； and one to bring along the wrong type of bulb.
　　If you’ve ever been disappointed by a workman when you really needed one， you might argue that the joke seems accurate.
　　Shared opinion
　　Sometimes the stereotype used is one that fewer people have had direct experience with and relies on a shared opinion. Here we have a joke based on the belief that psychiatrists[精神病学家] just say the same things to everyone， and don’t give any help.
　　How many psychiatrists does it take to change a light bulb？
　　Only one， but the light bulb must want to change.
　　Any group
　　Light bulb jokes， and satire in general， don’t just stick to jokes， though. They can make fun of any well-known group.
　　How many teenage girls does it take to change a light bulb？
　　One， but she’ll be on the phone for five hours telling all her friends about it.
　　But how do you feel if you are a teenage girl reading this？ Or a plumber？ Or a psychiatrist？ Satirical jokes are sometimes used to encourage negative attitudes towards groups of people， so listen carefully to the opening line before you join in the laughter.
　　Cute
　　On the other hand， they can be positive， or at least neutral[中立的]， like this one， which is based on some clever wordplay[字句的争论]， and is just cute.
　　How many babysitters does it take to change a light bulb？
　　None. They don’t make nappies small enough. Can you imagine a light bulb in a nappy？