Why do Britishers use stones to with things? I live in London and people say they weigh as much as a bunch of stones?

Update: I was talking to this fat women unfortunately and she toil dme she weighed 27 stones and needed to lose 10 stones. she looked like she was about 350 lbs. I told her she should lose 225 lbs and so I assume a stone is 13 lbs. How do they measure units less than a stone?
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