Knock Knock

A: "Knock Knock"

C: "Who's there?"

A: "Cow!"

C: "Cow who?"

A: "Moo!"

C: "Moo?"

A: "Quack!"

C: "If the answer is quack then we need to start over."

A: "Knock knock..."

This went on for half an hour this morning. They were cracking themselves up.