Tell me a joke

One night about a week ago, I was tucking Duncan into bed when he said, "Tell me a joke, Daddy."

So I told him this one:

A string walks into a bar and asks the bartender for a beer. The bartender says, "We don't serve strings here. Get out!" So the string leaves the bar, but he really wants a beer, so he gets an idea. He separates some of the fibers at one of his ends and twists himself up, then walks back into the bar and asks the bartender for a beer. The bartender says, "You're not a string are you?" To which the string replies, "No, I'm a frayed knot!"

Now Duncan insist that he be told that same joke every night. Tina tried to change it to a library instead of a bar, and it just wouldn't do!