"Universality"

One of my favorite things about physics is its universality. A hydrogen atom in the laboratory behaves exactly the same as a hydrogen atom on the back end of the Milky Way galaxy. Gravity is gravity is gravity; the force that keeps your feet planted to the ground is the same force that keeps planets in orbit around the sun, and that's the same force that has shaped the largest structures in the universe.

This key concept allows us to make enormous leaps in understanding. We can understand the state of the first few minutes of the Big Bang because we know how high-density plasmas work, so we can make testable predictions. We can figure out how stars die and blow up without having to visit one, because we can run a computer simulation of the fundamental physics.

Pretty handy, if you ask me.