One from the dark ages.
A ratty-looking string walks into a bar. The bartender looks at it and says, "Get out. We don't want you in here."
The string is annoyed at this, so as soon as it leaves, it twists itself up and goes right back into the bar. The bartender looks suspiciously at it and says, "Hey, aren't you the string that was just in here?"
"Nope," says the string, "I'm a frayed knot."