Re: Soaking the burnt pot in the hot water will weaken the stain and it will come out
Stains don't stick to pans. A stain is the mark left behind after something else has been removed. Burnt food might stick to a pan. If you soak the pan in hot water, the burnt food should come off but there may be a stain left behind.
I would say that a pan can become stained with/by the grease that was used to cook food in it (ie both prepositions are possible).
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