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Yo-yos are one of the most popular toys around, even after hundreds of years. See more toy pictures.

On the surface, the yo-yo is an incredibly simple toy -- it's really nothing but a spool attached to a length of string. But in the right hands, it can be something extraordinary: An accomplished yo-yoist can send the yo-yo flying out in all directions, make it hover in mid air, then snap it back into his or her palm. Ordinary string and wood (or plastic) are brought to life!

This may seem like magic, but it's actually just physics at work. Both the classic yo-yo and the sophisticate­d "automatic" yo-yos that have sprung up in the past few years are remarkable demonstrations of fundamental scientific principles.

In this article, we'll examine these principles to find out what makes yo-yos behave in such an unexpected way. We'll also look at the history of yo-yos and see how they've changed through the years.