Si is the yin-fire branch (hidden Bing, Wu, Geng), the 4th month, 9–11 am — warmth carrying ore within.
Si holds Bing at its peak and Geng's birth — where fire begins refining metal: intelligence turning into assets. Watch the Yin–Si–Shen punishment and Si–Hai clash; the Si–You–Chou metal frame is its cash-out path. Si turns heat into gold.