What is yellow and can't swim?

The joke “What is yellow and can’t swim? A school bus.” plays on the listener’s expectations. When someone hears the setup, they might initially think of various yellow animals or objects that can’t swim. However, the punchline reveals an unexpected answer: a school bus. The humor arises from the juxtaposition of the mundane reality of a school bus, which is obviously not meant to swim, with the initial expectation of something more animate or organic. The joke also touches on the dark humor of imagining a school bus, typically filled with children, in a situation where it would need to swim, thus creating a surprise element that catches the listener off guard.