Sand and Pebble Fountain

Dry sand can be made to exhibit fluid properties by blowing air through it from the bottom. If you do this to a tray of sand and add some pebbles to it, the pebbles will float to the surface pushed by the movement of the air (and the fact that they have a lower mass to cross-sectional area ratio than the sand). Also, the surface of the sand can be made to slope (and floating objects to float downhill) by varying the pressure of the blown air across the bottom of the sand. OK, enough theory.

The sand and pebble fountain would have a base layer of fluid sand and pebble mix made to slope from the middle to the outside. At the edge of the fountain the pebbles would be gathered by the slope of the rim and fed through a tube under the sand to the centre of the fountain. For there the pebbles would be thrown as high up into the air as wind and safety would allow, splashing back through the sandy surface of the fountain. They would then float to the surface, out to the edge, and the cycle would continue.