• Jon's body temperature increased after working outside in hot weather.
• Temperature-sensitive cells in the skin and brain detected the decrease in body temperature and sent a message to the brain.
• The thermoregulatory center in the brain received this information, processed it, and conveyed a message to the sweat glands of the skin.
• The sweat glands caused the body to perspire (sweat).
• The evaporation of sweat caused Jon's body temperature to decrease.
Question: What is the receptor (sensor)?