The valve plate is the material, usually flat or nearly so, that makes up the valve assembly. It needs to be durable and strong, and in many cases also resistant to heat, pressure and corrosion. There are valves that are automatically activated by changes in pressure within the setup of a machine. Other valves are manually operated through a lever, pedal or handle. In either case, the valve plate needs to be tough, non-corroding and resilient, because of its frequent exposure to varying pressures and temperatures. Some valves, like those in boilers and hot water systems, have to continually withstand temperatures well exceeding the boiling point of water.