The water faucet is an excellent example of designing a user interface.
In the beginning, there was only cold water.
On cold mornings, however, this was undesirable, so they added a second spout for hot water.
This still did not allow for temperature regulation, however. One could only get cold or hot, not warm water (unless one wanted to mix them in the sink, which required heating the entire sink, too). So they combined the two spouts into one.
The single spout with two handles is very common. It can be considered a decent solution. However, deep thought while in the shower will reveal that what one really wants is a different coordinate system: flow and temperature. Thus, the single rotating handle was born. Lift it to increase the flow, and rotate it to change the temperature.
This is still not perfect, however, because it is often difficult to move the handle in one direction without at least nudging it in the other direction. (I have used faucets where rotating the handle has no effect unless one also moves it up or down!) Thus, we arrive at the ideal solution: two handles, one which controls the flow (by opening both valves at the same time) and one which controls the temperature (by trading off between the hot and cold flows before mixing them).
Norman, Donald A. The Design of Everyday Things. New York, NY: Currency Doubleday, 1988.
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