Three strings are looking for a party (they're college strings after all), and decide to start their own. So, off to the convenience store they go. First string hops out of the car, saying he'll go get the beer. Walks into the convenience store, up to the register and says "I'd like to buy a six pack of beer please." Cashier eyes him up and down, and says, "We don't serve strings here."
String walks out all dejected, "He wouldn't sell me the beer." Second string chimes in and says, "Ya did it wrong, lemme give it a go." So, the second string walks in the store, walks back to the cooler, picks up a six-pack and says, "I wanna buy this six pack of beer." Cashier eyes him up and down, says, "Sorry, don't serve strings here." So, she walks out all bummed, saying "Cashier wouldn't sell me the beer either."
So, the third string chimes in and says, "Both y'all screwed up, I'll get it." Gets out of the car, goes back to the cooler, brings it up, tosses enough money on the counter to cover the beer and starts to walk out the door. The cashier shouts out, "Hey, ain't you a string?!?!"
String says, "Nope, I'm a frayed knot!"