Where the shape comes from
A lump of rock lying at the foot of a cliff: the overhang 厂 above, the block 口 fallen beneath it. It marks stone and everything worked from it or ground with it: 硬 hard, 碗 a bowl, 碎 to smash, 破 to break, 码 a number set in stone. Read dàn it is an old measure of grain, still met in older texts.