Sensor
A sensor is something that detects a change. Sensors detect many things:
Light: Light sensors detect when it's dark and turn on lights or you walk down a path at night the sensor turns on a light for you. This is called a motion sensor.
Motion: Motion sensors detect when someone is moving and can turn on lights or open doors.
Temperature: Temperature sensors detect when it's cold and turn on the furnace or if it's hot turn on the air conditioner.
Proximity: Proximity sensors detect when something is near.
Rain: Rain sensors detect when it's raining or snowing and turn on windshield wipers.
Smoke: Smoke sensors detect smoke and can turn on fire alarms or fire extinguishing systems. These are very important sensors. They can save your life.
Gas: Gas sensors detect gases like LPG, propane, butane, and methane