Newton’s third law of motion says that for every action there is an equal reaction in the opposite direction. The spinning cup in the diagram is an example of this law.
The Action-Gravity forces the water in the cup out of the straws in the cup. The direction of the water is determined by the direction the ends of the straws are pointing. In the diagram, the water is squirting out of the straws in a clockwise direction.
The Reaction- The cup spins in the opposite direction of the action, thus it spins counterclockwise.
The rotating cup is named after its inventor, Hero of Alexandria (around 60 A.D.)
You can make your own Hero’s Engine using the procedure on this page, PHYSICS: HERO’S ENGINE