The physical world is composed of many forms of energy that we perceive in the form of light, heat, sound, or motion. While driving a car when we are near traffic lights and see a red light up ahead, we start to slow down and stop the car. We are just sensing a light form of energy and this led us to change our activity because someone taught us what every traffic light's phase means.
This natural behavior is the same we want to bring to our interactive physical applications. We use hardware devices called sensors that listen to the environment and that cooperate with other hardware components, called actuators, which perform actions in the real world. However, we need a third element called microcontroller that uses connected sensors and actuators to sense and alter the surrounding environment, according to the uploaded program.