The sum of potential and kinetic energies in the particles of a substance is called energy.

Explanation:

This sentence is the description of a specific type of energy: the mechanical energy.

To unerstand it better:

The mechanical energy of a body, a system or a substance is that which is obtained from the speed of its movement (kinetic energy) or its specific position (potential energy), in order to produce a mechanical work.   This means mechanical energy involves both the kinetic energy and the potential energy (which can be elastic or gravitational, for example).  

In addition, it should be noted that mechanical energy is conserved in conservative fields and is a scalar magnitude.

Therefore:

The sum of potential and kinetic energies in the particles of a substance is called Mechanical Energy

blue light is scattered in different directions by the tiny molecules of air. blue is scattered more than other colours because it has a shorter distance to travel which results in smaller waves. this is why we see a blue sky most of the time.