Can a filthy thing be used to clean? Of course not!
The Prophet went out to answer the call of nature and asked me to bring three stones. I found two stones and searched for the third but could not find it. So took a dried piece of dung and brought it to him. He took the two stones and threw away the dung and said, “This is a filthy thing.”
The word ‘stones’ here does not mean rocks. It’s used for the dried and hardened ‘stones’ of soil.
And the rule is simple: only a clean thing can be used for cleaning, e.g. water, soil. Dung and bones should not be used. More reasons for this are yet to come..