Another concept in tai chi is that the forces of yin and yang should be in balance. In Chinese philosophy, yin and yang are two principles or elements that make up the universe and everything in it and that also oppose each other. Yin is believed to have the qualities of water--such as coolness, darkness, stillness, and inward and downward directions--and to be feminine in character. Yang is believed to have the qualities of fire--such as heat, light, action, and upward and outward movement--and to be masculine. In this belief system, people's yin and yang need to be in balance in order for them to be healthy, and tai chi is a practice that supports this balance.