n. 【商标】约约〔一种用线扯动使忽上忽下的轮形木制玩具〕；〔美俚〕迟钝愚蠢的人。 vi. 玩约约；忽上忽下。
a toy consisting of a spool that is reeled up and down on a string by motions of the hand
The yo-yo in its simplest form is an object consisting of an axle connected to two disks, and a length of twine looped around the axle, similar to a slender [It is played by holding the free end of the string (usually by inserting one finger in a slip knot]) allowing gravity or the force of a throw to spin the yo-yo and unwind the string (similar to how a [[pullstring works), then allowing the yo-yo to wind itself back to one's hand, exploiting its spin (and the associated rotational energy). This is often called "yo-yoing".
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