Pedro Flores introduced the Yo-YoŽ to the US in 1928 and, after seeing the Yo-YoŽ in action in San Francisco in 1929, Donald F. Duncan bought the idea from Flores and trademarked the name. Duncan is credited with introducing the slip string which allows the Yo-YoŽ to "sleep" and the introduction of the butterfly shape (reversed edges of the tradional imperial shape) allowed Yo-YoŽ enthusiasts to develop more tricks.
The Yo-Yo (which means "come back" or "return" in the native language of the Philippines) is over 3000 years old and considered the second oldest toy in history. Second only to dolls!