Thermodynamics

Everything so far, you could watch one thing at a time — a particle, a ship, a planet, an electron. Now we lose that luxury. There are simply too many.

What is heat? For a long time people thought it was a fluid — "caloric" — that flowed out of hot things into cold ones. The first real crack came in 1798, when Count Rumford, supervising the boring of cannon barrels for the Bavarian army, noticed the brass stayed hot for as long as the drill kept turning. No fluid could be that bottomless. The heat, he argued, was coming from the motion of the boring itself. Through the 1800s, Clausius, Maxwell and Boltzmann made the idea exact: heat is just motion. Trillions of tiny atoms, jittering and slamming into everything. Hot is fast; cold is slow. That's all it ever was.

You can't possibly follow trillions of anything. So you stop tracking individuals and start measuring the crowd:

• Temperature — how fast they move, on average.
• Pressure — how hard, all together, they hammer the walls.

The molecules on the right are doing the hammering. Squeeze them into less room — slide the piston in — and watch the pressure climb.