The magician shows a pair of magic wands. Each wand has a red string through one end with a tassel tied to it. One of the tassels is one a long string, and the other is on a short string.
When the magician pulls on the short-stringed tassel, that string gets longer, but the string on the wand with the tassel on a long string gets shorter at the same time. When the now long (formerly short) stringed tassel on the wand that had the short (now long) tassel is left hanging and the newly short tassel is pulled, the first short tassel becomes again the short tassel while the long tassel is, likewise, long.
This fascinating effect is repeated repeatedly with the wands held in different places (hand, pocket, underarm, mouth, pants) and sometimes with spectators pulling the short tassel or moving metal hoops around the wands to prove that there are no strings.
Although the wands appear to be separate, they are really attached by a thick piece of tassel string. Although great lengths are gone to to "prove" that the wands are not attached, they really are, and when the magician says that they aren't it's a complete lie. In fact, anybody who's paying attention can see the big red string going between the wands, but magicians are so trusted that nobody even thinks to look.
By the way, magicians lie all the time. "My assistant is gone!" "There's a quarter in your ear." "This rabbit is completely solid." "That wasn't me with your girlfriend." All lies. It's tragic. Don't get me started.