Knock Knock / Who’s there? / A Pile-Up

Knock Knock
Who’s there?
A Pile-Up
A Pile-Up Who?
Ewwwwww

This joke combined two of the highest forms of humor: the knock knock joke, and bathroom humor.

The critical part of this joke is the fact that "A Pile-up Who" sounds like "A pile of poo", which is a funny thing to say because it is shocking to be talking about poo (at least to little kids anyway.)