Like home heating system, an animal achieves homeostasis by maintaining a variable, such as body temperature or solute concentration, at or near a particular value, or set point. Fluctuations in the variable above or below the set point serve as the stimulus detected by a receptor, or sensor. Upon receiving a signal from the sensor, a control center generates output that triggers a response, a physiological activity that helps return the variable to the set point. In the home heating example, a drop in temperature below the set point acts as a stimulus, the thermostat serves as the sensor and Control Center, and the heater produces the response.