Mass--energy equivalence

In physics, mass--energy equivalence is the concept that the mass of an object or system is a measure of its energy content. For instance, adding 25 kilowatt-hours of any form of energy to any object increases its mass by 1 microgram, increasing its inertia and weight accordingly, even though no matter has been added. This video targeted to blind users. Attribution: Article text available under CC-BY-SA Creative Commons image source in video