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.
The simplicity. Just two sentences can create a whole scene and a laugh. For example, 'I told my dog he was adopted. He still doesn't believe me.' It's easy to understand and quick to deliver the humor.