Daniel supposed that was all there was to that, until he went with Hooke to collect samples of river insects one day, and they squatted in a place where a brook tumbled over the brink of a rock into a little pool. Bubbles of water, forced beneath the pool by the falling water, rose to the surface: millions of tiny spheres. Hooke noticed it, pondered for a few moments, and said: 'Planets and stars are spheres, for the same reason that bubbles and sparks are.'