The third and fourth graders liked this one.
A string was walking down the street. Realizing he was hungry he went into a Rally's Hamburger restaurant. At the counter he ordered a hamburger and coke. The man behind the counter asked, "Aren't you a string?. We don't serve strings here. You'll have to leave". The string continued down the street. Still hungry he entered a Burger King. At the counter he asked for a hamburger and coke. He was told again strings were not served there.
Now he is really hungry. Maybe if he changed his appearance he may get served. So, he tied himself in a knot and frayed out his top to make it look like hair. Then he arrived at McDonald's. He ordered a hamburger and a coke. The young woman behind the counter looked him over and asked, "You're a string aren't you?", to which he replied "I'm a frayed knot".