E = mc2, the world’s most famous equation, is 100 years old. According to this BBC article:
Einstein showed in a handful of lines that as you accelerate an object, it not only gets faster, it also gets heavier.
That in turn makes further pushing less fruitful so that eventually nothing can be accelerated beyond the speed of light.
Sure, sure, sure . . . but the really cool thing about that equation is that c2 — the speed of light squared — is a really BIG number; hence, you can get a LOT of energy (e) out of a very small amount of mass (m). This has tremendous practical implications, as was later demonstrated in spades by the development of the atom bomb.