No. 70: Jul-Aug 1990
The next time you walk along an ocean beach, forget the greater anomalies of nature and pick up a few well-worn pebbles. Q.R. Wald collected 200 such at random and measured them with calipers. He found that their three axes tended toward the ratios 7:6:3. No, beach pebbles are not spherical, they are flattened ovoids. We would naturally expect that the ceaseless action of the surf would turn out nearly perfect spheres. So much for intuition!
(Wald, Quentin R.; "The Form of Pebbles," Nature, 345:211, 1990.)