A calm river suddenly starts churning in the middle of a flat stretch: that's the puzzle the authors of a new study set out to solve. They took a thin layer of charged gas with a very weak magnetic field and tracked on a computer what would happen to it. At first, everything flows smoothly, like water from a tap. Then, in one spot, the layer thins out, and the magnetic field lines, which previously lay parallel, begin to snap and reconnect.

This reconnection is like a rubber band in a tangle of yarn snapping, its ends whipping in opposite directions. Each such snap injects a bit of chaos into the flow. At first the chaos is weak, but the snaps repeat, and soon the whole flow is boiling: vortices appear, eddies, tiny jets. And no one pushed the flow: it stirred itself up.

This matters for understanding the cosmos. The solar wind, plasma jets from the vicinity of black holes, comet tails — all of them are turbulent. Previously, scientists thought turbulence came from outside. Now we see it can be born from within, out of a smooth flow, thanks to magnetic reconnection.