Ridley’s first joke:

R(idley): Knock, knock
M(e): Who's there?
R: An owl
M: An owl who?
R: Whooooooo!

During the first exchange Ridley & I had of this preschool joke, I wondered whether his punchline of "whoooo" was a mere mimic of the word "who" I'd just spoken. However, he's since repeated this joke to me four times over two days, so I'm beginning to suspect that there's actually humourous intent on his part. Amazing.