The lines
The I Ching’s two ingredients are two kinds of line:
broken  (yin)
and solid  (yang)
These are the deep roots of a traditional Chinese idea: the relationship of yang and yin gives rise to all that is.
Yang and yin aren’t fixed; they’re ways of relating, and they only exist in relation to one another. You can see that this is true from the modern Chinese characters for yin 陰 and yang 阳: yin shows the shaded side of a hill, yang shows the sunny side. You can’t have half a hill, and you can’t have yin without yang or yang without yin.