We should have heart of rubber. Rubber is soft. Rubber is stretch. If rubber receive strong pressure, rubber ease the pressure and absorb the pressure. So,we should have soft heart. If soft heart receive stress, soft heart ease stress and absorb stress. Soft heart means mild and modesty. We should not have heart of stone. Stone is hard. If stone receive strong pressure, stone is broken. So, if heart of stone receive stress, heart of stone is broken. Namely, heart of stone means obstinate and arrogance.