Inventor James Dyson's fountain, inspired by the work of MC Escher, gives the illusion that its water flows uphill:
Covering the ramp is a glass surface. Water is pumped in at the bottom, and comes out of the opening at the top. At the opening, some of the water is diverted back down the ramp, covering the glass in a thin layer of water.
Compressed air is also pumped in where the water enters - bubbles then travel up the ramp to the opening. These bubbles, combined with the thin layer of water going downhill, are what create the illusion that the surface of the ramp is not just a glass lid.
Link via OhGizmo!
Ive seen a fountain very similar to this but with only one ramp. The illusion is pretty convincing.