mass–energy equivalence (E=mc²)
Mass–energy equivalence is Einstein's discovery that mass and energy are really two faces of the same thing — you can turn one into the other. A tiny bit of mass holds an astonishing amount of energy, because the formula multiplies it by the speed of light squared (c²), and that number is colossal.
How colossal? The mass of a single paperclip, fully converted, could power a home for centuries. This is why the Sun shines: deep in its core it fuses hydrogen, and a sliver of the mass goes missing — released as the light and warmth that reach your skin. The same trick powers nuclear reactors and, less happily, nuclear weapons.
One honest caveat: in everyday life mass and energy still feel like separate, well-behaved things, and they are — the conversion only becomes dramatic at the heart of stars or inside a reactor. But the books always balance: nothing is truly lost, only changed from one form into the other.
Energy equals mass times the speed of light, squared.
The c² isn't there because anything moves at light speed — it's just a conversion factor between the units of mass and energy. But because c is so enormous, c² turns a pinch of mass into a flood of energy.