Even for something as pedestrian as pouring cream into coffee, the equations that govern the fluid dynamics are simply too difficult to solve, even with a powerful computer. Little motions that deviate from computer-model predictions quickly balloon into eruptions and vortices that can end up dominating the whole flow. In fact, some scientists would argue that more is known about the interior of a black hole—a region where no experiments have ever been conducted and, even if they were, their outcomes could never be observed—than about cream mixing into coffee. Or smoke from a snuffed-out candle. Or a gust of wind.

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