A very complex machine. It is not one that you can turn on and off. Every engine requires fuel, when the fuel runs out the machine will stop. The human body, too, is a machine and continuously need to be refueled. Unlike man made machines it can not be turned on and off.
Energy is defined as the capacity to do work and is the power that keeps the body functioning.
It is perhaps the most frequently discussed topic in nutrition. Some even resort to starvation. Just like fat, proteins and carbohydrates are deposited into fat cell if the intake is too much, but just like carbohydrates, fats and proteins are used for energy. Closely associated with the goal to achieve desirable body weight is the participation by millions in exercise programs. About 500 million people in the world do not get enough food to meet their body's energy needs.