Have you ever seen a clock with a liquid display? Rhei is an electro-mechanical prototype clock in which the hours are displayed using ferrofluid. This is a liquid that becomes magnetised in the presence of a magnetic field. Ferrofluid was given its name because it fuses the characteristics of ferromagnetic elements (magnet) and fluid substances (water or organic solvent). When the magnetic field approaches the fluid, a conflict occurs in the material, at the same time the elements are attracted by the magnets and the oily solution tries to stay static, which causes an impressive effect on the display.
Here is a link to a really cool video about how this clock works and how its made - click here to view video.