Renzi (壬子) and Guichou (癸丑) are associated with the Mulberry and Zhe Wood. Zi (子) belongs to Water, and Chou (丑) belongs to Metal. Water nourishes Wood, while Metal cuts it down. Just as the mulberry and zhe trees grow, people use them to feed silkworms, hence they are called Mulberry and Zhe Wood.