As we know sun is the biggest natural source of energy. We can also get energy from the nuclei of atoms, the interior of the earth and the tides.
But in science, we give energy a definite and precise meaning. A fast moving cricket ball hits a stationary wicket, then the wicket is thrown away, why?
A raised hammer when falls on a nail placed on a piece of wood, it drives the nail into the wall. Why ?
When a balloon is filled with air and we press it, we find a change in its shape. Why?
In all of the above, the objects, through different means, acquired the capability to work. So an object having the capability to do work is said to possess energy.
The object which does the work loses energy and the object on which the work is done gains energy.
In science, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is defined as the ability of an object to do work.