# A simple sink a simple looking faucet but it looks as

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found someone who could explain the taps to her. A simple sink, a simple-looking faucet. But it looks as if it should be turned, not pushed. If you want the faucet to be pushed, make it look as if it should be pushed. (This, of course, is similar to the problem I had emptying the water from the sink in my hotel, described in Chapter 1.) Why is such a simple, standard item as a water faucet so difficult to get right? The person using a faucet cares about two things: water temperature and rate of flow. But water enters the faucet through two pipes, hot and cold. There is a conflict between the human need for temperature and flow and the physical structure of hot and cold. There are several ways to deal with this: Control both hot and cold water: Two controls, one for hot water, the other cold. Control only temperature: One control, where rate of flow is fixed. Rotating the control from its fixed position turns on the water at four: Knowing What to Do: Constraints, Discoverability, and Feedback 151 some predetermined rate of flow, with the temperature controlled by the knob position. Control only amount: One control, where temperature is fixed, with rate of flow controlled by the knob position. On-off. One control turns the water on and off. This is how gesturecontrolled

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faucets work: moving the hand under or away from the spout turns the water on or off, at a fixed temperature and rate of flow. Control temperature and rate of flow. Use two separate controls, one for water temperature, the other for flow rate. (I have never encountered this solution.) One control for temperature and rate: Have one integrated control, where movement in one direction controls the temperature and movement in a different direction controls the amount. Where there are two controls, one for hot water and one for cold, there are four mapping problems; Which knob controls the hot, which the cold? How do you change the temperature without affecting the rate of flow? How do you change the flow without affecting the temperature? Which direction increases water flow? The mapping problems are solved through cultural conventions, or constraints. It is a worldwide convention that the left faucet should be hot; the right, cold. It is also a universal convention that screw threads are made to tighten with clockwise turning, loosen with counterclockwise. You turn off a faucet by tightening a screw thread (tightening a washer against its seat), thereby shutting off the flow of water. So clockwise turning shuts off the water, counterclockwise turns it on. Unfortunately, the constraints do not always hold. Most of the English people I asked were not aware that left/hot, right/ cold was a convention; it is violated too often to be considered a convention in England. But the convention isn t universal in the 152 The Design of Everyday Things United States, either. I once experienced shower controls that were placed vertically: Which one controlled the hot water, the top faucet or the bottom?
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