A classic of magic.

The magician shows two sticks, each with a cord and tassel running through its end. The are held close together, one long string only hanging down. The magician pulls on the short string, apparently pulling the string right through the ends of the sticks, as the long string becomes shorter and the short string becomes longer. But even when the sticks are separated, either held parallel or crossed one over the other, when the short cord is pulled out the long cord shortens.