The Chinese and Japanese imitated nature in their water gardens. The water garden was a sign of perfection and peace. By the third century AD they had written down the essentials of a water garden. In the 15th century, they turned the water garden into an art form that used the sound and motion of water. A reflecting pool expressed the union of heaven and earth. The Japanese built a dry garden, using small rocks, sand, and other materials to represent the course of a stream or river. Nature was recreated to serve as a source of thinking.