Rock, sand and gravel are an essential feature of the Japanese garden. A vertical rock may represent Mount Horai, the legendary home of the Eight Immortals, or Mount Sumeru of Buddhist teaching, or a carp jumping from the water. A flat rock might represent the earth. Sand or gravel can represent a beach, or a flowing river. Rocks and water also symbolize yin and yang ("in" and "yō" in Japanese) in Buddhist philosophy; the hard rock and soft water complement each other, and water, though soft, can wear away rock.