Tibetan Buddhists developed the prayer wheel as a substitute for the repeated recitation of mantras (syllables or verses of mystical power).
The wheel is a hollow metal cylinder mounted on a rod, with the mantra written on a roll of paper inside the cylinder.
The Buddhist turns the wheel by hand; each turn is believed to have a spiritual effect equivalent to reciting the prayer orally.
Prayer wheels are sometimes mounted on windmills and watermills, where natural forces can, in effect, be put to work “reciting” the mantra.