Snakes can have dozens of young at a time, and so they are often symbols of fertility. They resemble vegetation, especially roots, in their form and often in the green and brown of their skins. The undulating form of a snake also suggests a river. A point of muscular tension passes through the body of a snake and drives the animal forward, like a moment moving along a continuum of days and years. Like time itself, a snake seems to progress while remaining still. In addition, the body of a snake also resembles those marks with a stylus, brush, or pen that make up our letters. Ornamental alphabets of the ancient Celts and others were often made up of intertwined serpents. It could even be that the tracks of a snake in sand helped to inspire the invention of the alphabet. The manner in which snakes curl up in a ball has made people associate them with the sun.