Why will a light ping-pong ball dance over a stream of gas or water issuing at high speed from a tube ?

Q: Why will a light ping-pong ball dance over a stream of gas or water issuing at high speed from a tube ?

Sol: Since the stream has a high velocity , the pressure inside the stream is below the atmospheric pressure . The ball is supported from the bottom by the thrust of stream and kept in position by the lateral pressure difference .