Dutch designer and self-described "formfinder" Arnout Visser has incorporated a keen observation of fluids into the design of his products. His Archimedes letter scale, seen above, is as simple and elegant as it gets: Place a letter on the floating glass pedestal on top, the pedestal sinks and the water level rises accordingly.
His Salad Sunrise cruet combines the oil and vinegar into one container. As the former is lighter than the latter, the two fluids never mix; placing spouts on opposite sides and at the appropriate heights allows you to pour them separately.
The Water Loep is a simple magnifying glass consisting of a blown glass form filled with water, which provides the magnification.
Check out more of Visser's stuff here.