Gravel is usually used in zen gardens, rather than sand, because it is less disturbed by rain and wind. The act of raking the gravel into a pattern recalling waves or rippling water, known as or , has an aesthetic function. Zen priests practice this raking also to help their concentration. Achieving perfection of lines is not easy. Rakes are according to the patterns of ridges as desired and limited to some of the stone objects situated within the gravel area. Nonetheless, often the patterns are not static. Developing variations in patterns is a creative and inspiring challenge.