This is a simple trick that can be performed very rapidly. Father and child make a loop of string and thread a ring on it. Ask someone to extend his forefinger upward, and slip one end of the loop over each finger, so the forefingers hold the string with the ring between them. The problem is to remove the ring without taking the string from the person's fingers. To do this, take hold of one string to the right of the ring, and slip the string over the person's right finger, which will be to your left. Immediately remove the loop that was already over the right finger, and the ring will fall off, but the string will run from finger to finger.