It is impossible to talk about yin or yang without some reference to the opposite, since yin and yang are bound together as parts of a mutual whole (for example, there cannot be the bottom of the foot without the top). A way to illustrate this idea is to postulate the notion of a race with only women or only men; this race would disappear in a single generation. Yet, women and men together create new generations that allow the race they mutually create (and mutually come from) to survive. The interaction of the two gives birth to things, like manhood. Yin and yang transform each other: like an undertow in the ocean, every advance is complemented by a retreat, and every rise transforms into a fall. Thus, a seed will sprout from the earth and grow upwards towards the skyan intrinsically yang movement. Then, when it reaches its full potential height, it will fall. Also, the growth of the top seeks light, while roots grow in darkness.