Red Paper Heart, an art studio working in interactivity and animation, was asked to make a pool "interactive" for a party. Since the surface of water is not an easy medium easy to project upon (especially when people are swimming in it), they poured in 65,000 ping pong balls to float on the surface and reflect the projected light! The balls are fairly easy to swim through, and even when moving provided a stable projection surface. You can see a series of photographs of the project at their website. Link -via mental_floss
I think Cameron did the ping pong ball thing with black Styrofoam peanuts when he needed to make daylight exterior pools look like the dark bottom of the ocean for parts of 'The Abyss.'
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