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  "body": "\n## Summary\n\nA dynamical billiard is the motion of a point particle in a region, called the table.\nThe particle moves in straight lines at constant speed.\nAt the boundary it bounces, with angle of reflection equal to angle of incidence.\nThe shape of the table alone decides the behaviour: some tables give regular, integrable motion, others give chaotic, ergodic motion.\nBilliards are model systems for the ergodic hypothesis of statistical mechanics and for quantum chaos.\nThe [[Sinai billiard|GX9Q2PYNZE]] and the [[Bunimovich stadium|YYB1JP7TWJ]] are the classical chaotic examples.\n\n## Explanations\n\n### L1 human\n\nThink of a billiard ball with no friction and no spin on a table of some shape.\nIt rolls straight, hits the cushion, and leaves at the same angle it came in, like light on a mirror.\nOn a round table every shot keeps the same angle to the cushion forever, so the motion is very regular.\nOn a table with a round obstacle in the middle, tiny changes of the first shot lead to completely different paths, and the motion is chaotic.\n",
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