Knock Knock Joke

So, this is totally trivial, but my six-year-old sister called to tell me this joke the other morning before she left for school. I've been laughing about it all week long. :-D

Knock Knock?

[Who's there?]

Centipede

[Centipede who?]

Centi - pede on a kid's lap!

Get it? Get it??

Here's a clue in case you don't. (now we really know why I was crying.)