A wind of about ten and a half miles per hour will start to move dry sand, and ripples are the tiny, more or less parallel ridges of migrating sand that result. A ripple is to sand what a wave is to water, and just as a series of nearly identical waves may move across the surface of the sea, so a series of nearly identical ripples may crawl across the windward face of a dune. Reaching the crest, the ripples spill their contents down the leeward side, or slip face, of the dune, feeding its downwind migration. Stream currents or wave action on a shore may also cause bottom sand to ripple. Water, of course, also ripples. Born from wind, eddying current, tossed stones, or a thousand other varieties of disturbance, ripples radiate across otherwise still surfaces of lakes, ponds, and rivers. They also radiate across the surface of our thoughts, where they abide as the metaphor and epitome of lingering effect and continuing implication.