One of my friends and I have developed a habit of drinking water from only copper vessels as I have read several benefits of drinking from that. But I observed one strange phenomenon during past few months. Even if both of us wash our vessels on the same day still my vessel gets darker and more stained than his, almost always within a day or two. On the other hand, his vessel keeps looking shining and less stained. I know these stains are results of oxidation, but still why do they react differently being with different people ?
[ PS : We had bought our vessels on same date from the same shop, both of us stay in the same dorm room, eat the same food, and have very similar living habits ]