Sinai BilliardA ball bounces forever inside a square with one round pillar at its center. Launch a hundred nearly-identical balls together and watch: every bounce off the curved pillar fans the bundle out a little more, and within a few hits they scatter across the whole table — pure cause-and-effect motion that looks just like a random gas.Billiards look like the simplest possible mechanical system — one point mass, straight lines, elastic bounces, zero randomness — yet Sinai proved in 1970 that adding one convex obstacle is enough to make the motion as statistically random as a gas of colliding molecules. The mechanism is purely geometric: a curved wall defocuses a bundle of nearby trajectories instead of keeping them parallel. This exact defocusing argument convinced physicists that deterministic chaos, not literal randomness, can be the real origin of the statistical assumptions baked into thermodynamics.