c represents the speed of light, which is a constant value, and therefore, c is a constant. Light, unlike you, doesn't become tired and slow to a near crawl after ever quarter lap. It is rather tireless. E and m, on the other hand, are variables representing energy and mass, respectively. In other words, the greater an object's mass, the greater the amount of energy. Taking your gargantuan, couch-dwelling Uncle Leroy into consideration, this information is actually hard to believe.