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Scientists studying the rounded rocks on the surface of the asteroid 25143 Itokawa, photographed by the Japanese probe Hayabusa, have concluded that sunlight combined with the asteroid’s tiny gravity caused them move and thus erode themselves.
As sunlight bounced off the orbiting boulders, photons provided a tiny push. As they radiated back outward as heat, they triggered a recoil effect that added a gentle spin. Over time, these slowly spinning boulders bumped into each other with enough force to wear their edges into smooth surfaces.
Warning! This is only a model, and thus could very well be wrong. It is reasonable however and worth considering as a factor in studying the early formation history of asteroids.