"Introducing Science" by Alan Isaacs, p.128:

..Matter is associated with energy in two distinct forms, that is, energy of motion, called kinetic energy, and stored energy, called potential energy. Each of these two forms of energy contribute to the mass which is the characteristic property of matter. In the case of an electron, part of the mass results from its electric charge (this is the potential energy) and part is contributed by the kinetic energy due to its movement. The greater the speed of movement, the greater the kinetic energy, and consequently the greater the mass. In fact, an electron moving at 99% of the velocity of light is seven times heavier than an electron at rest.

Interesting!