What it shows
Bernoulli's principle shows the velocity dependence of pressure in a fluid. Here, fast flowing air creates a zone of low pressure that holds a beach ball aloft.
How it works
Here we have a beach ball held in the air stream from a powerful source (whose original function was an industrial furnace air blower, see Comments). The ball is locked in position by the air flow surrounding it; the fast moving air is at a lower pressure than the still air; should the ball move from the stream, the higher pressure of the still air forces it back in. The pull of gravity is balanced by the force of the rush of air, leaving the ball completely stationary in space. The stability is such that the ball can be pushed out of place and it will return to its equilibrium position.
Setting it up
The blower can be set at any angle, but that angle is invariably 45°. Plug it in and place the ball in the air stream. A good one to have running as the audience files in.
It is instructive to ask the audience which way the ball will fall when the blower is turned off. "Straight down" is not correct; it will in fact fall down the air stream at the 45° angle.
The blower, manufactured by ILG Electric Ventilating Co., Chicago 41 is 1/4 h.p. and runs at 3400 RPM. A collimating nozzle and safety cage over the fan have been added, and it has been mounted on a simple wooden hinged base. A smaller blower such as Air Source® or inverted vacuum cleaner will raise a ping-pong ball well, but you need more wind to lift anything bigger.