The Chinese zodiac works on... let me think about this for a sec... branches and stems? Something like that. Anyway, the gist of it is, there are 12 signs, and 5 different elements (or colors, depending on your point of view). Thus, 12x5 gives you a 60 year cycle. However, I seem to recall reading there is an even deeper series of cycles, like 120, but I don't know much about that...