What will cause a yo-yo with a brain to return to the holders hand?

This is a joke. Think: Why would a yo-yo with a brain return to the holder's hand? Of course, if he decided not to return, how would he do it? Break the string.

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A yo-yo with a 'brain', is simply a yoyo with springs on the side disks, which when thrown with enough force, will expand due to the centrigual force exerted in the disks from the quick rotation, and free the yoyo from its stationary hold on the string.

When the yoyo's rotation slows down, the centrifugal force decreases, and so the springs return to normal, which will lead to the yoyo catching the string again, to return to the holder's hand.