They have associations with real world in many different layers, most popular four of them are:

1. Time of the day
2. Period of the year
3. Moon phases
4. Vegetation growth cycle

Ancient Chinese new that every object in the Universe that reached its perfection (extreme form) will unavoidably start transitioning itself to its opposite and then morphing back to its previous stage, but in the different level. For example, the early morning time progresses to the noon and then thru the afternoon, to early and late evening and then to the night to become next day's another morning. But this is a morning of the other day – not the same day. The same cycles you can easily find in the seasons of year, moon phases and life cycle of plants.