Isaac's first joke

Isaac has not let his limited vocabulary stop him from telling his first joke.  It happened last night when his two older brothers were telling knock-knock jokes at dinner.  It went like this:

Isaac: *grunt grunt*
Me: Who's there.
Isaac: Boo
Me: Boo who?
Isaac: *grunt grunt grunt grunt grunt* Ha ha ha ha!

In case you don't speak Isaac, he was telling the classic kid's joke that goes:

Knock, Knock
Who's there?
Boo
Boo who?
Don't cry, it's only a joke.

He thought it was funny (and I'll admit I thought it was funny too), so he continued to tell the joke continually through dinner.  It was the same joke over and over and over.  But that's how kids are right?  They think if it's funny the first time, it's still funny the 100th time.