If terrace steps on the surface were just oriented randomly across the pearl, this would average out to zero. But the terraces are arrayed in parallel like lines of longitude on a globe, creating a ratchet-like profile around the circumference of the pearl. Because of this ratchet shape, the small kicks imparted by warmed water molecules act in the same direction, causing the growing pearl to rotate. The researchers estimate the size of this force to be roughly 0.1 newtons – about the weight of a strawberry – for a pearl of 1cm in diameter, which should produce a rotation rate more or less equal to that observed.
The researchers admit that there are still gaps in their argument, but they say that the idea might be applied to make little spontaneously rotating motors. But don’t worry: they haven’t invented perpetual motion. The rotation is powered by the heat released during the chemical process of crystallisation, and it will stop when there is nothing left to crystallise – in other words, when the “fuel” runs out.