Traditional faucets use a rubber screw-down valve and a rubber washer to control the flow of water.  Newer ceramic disk valve faucets can be turned completely on or off with the flick of a wrist, but they are susceptible to mineral scale damage in hard water areas.  Bathtub and shower faucets often feature a thermostatically controlled, antiscald valve option.  Hands-free faucets are more hygienic than traditional faucets because they rely on infrared sensors to turn the water flow on and off.  At one time, sinks had separate hot water and cold water faucets, but most faucets today have a single spout serviced by both hot and cold water lines.  Either two separate valves or a single lever may be used to control the flow of hot and cold water, thereby adjusting the temperature of the combined water flow.