Yellow and Can't Swim

Alright, listen up, buttercup. The joke is a play on words. You thinkin' of somethin' yellow like a ducky or somethin', right? But BAM! It's a school bus full of kids that can't swim 'cause, well, they're at the bottom of a lake or somethin'. It's funny 'cause it's dark and you didn't see it comin'. You probably thought school buses were just for gettin' to school.