Alright O+W=H, you're old enough to remember Clackers - two acrylic balls (about the size of a golfball), each on the end of a string. The two strings were about 6 inches long and connected.
The object was to hold the strings where they came together and move your hand up and down so the two balls collided with each other below and above your hand.
Probably responsible for more injuries than any other toy with the possible exception of Jarts (lawn darts).
Injuries resulted from hitting yourself in the hand, arm, face, etc.; from using Clackers like a bola; and from the balls shattering!
Ah, the good old days...