The contrast. For example, when you start with something normal like 'My neighbor always gives me strange looks' and then follow with 'Today I found out he was looking at the ghost behind me'. The normal - to - spooky shift makes it effective.
A snail got robbed. The police asked, 'Did you get a look at the assailant?' The snail said, 'No, it all happened so fast.' For the second one, a bee went to the hairdresser. The hairdresser asked, 'How do you want your hair?' The bee said, 'Just make it look like a wasp, please.'
A man walked into a library and asked for a burger. The librarian said, 'This is a library.' The man whispered, 'Sorry. Can I have a burger?' Another one: A magician was walking down the street and turned into a grocery store.
A duck walked into a store. It asked for some bread. The cashier said ducks don't use money. So the duck left its feather as payment. The cashier was so amused that he gave the duck the bread.
A squirrel entered a library. It climbed on a bookshelf. It started reading a book about nuts. The librarian was shocked. But the squirrel just continued reading calmly.
The other day I told my wife she was drawing her eyebrows too high. She looked surprised. This is funny because when you say someone is drawing their eyebrows too high, the visual image of them looking perpetually surprised is quite comical. Also, the simplicity of the statement and the quick reaction it implies from the wife add to the humor.