If you are having a lazy afternoon and want some amusement, why not get a faucet and a ping pong ball and start a juggling act? That's what researchers Roberto Zenit and Enrique Soto did. They tossed ping pong balls into the stream, and recorded the balls getting captured by the water.
The water forms a moving film around the ball. If the ball tips one way, it will have to push against that film to escape. The water will push back, shoving the ball back into place. If, on the other hand, the water film goes out of balance, another effect kicks in. The researchers calculate that more water on one side of the ball will make for increased water speed on that side of the ball. According to the Bernoulli Principle, the increased speed will decrease the pressure on that area of the ball. The rest of the water film will then push the ball back into place.
Via Physics Central.