I was told years ago that the aluminum we use daily isn't really pure aluminum, it's mixed with other metals. And there are minerals in the water, and in the foods we cook. With all those mixes of minerals, some are sure to react with some of the others.

The guy that told me this (a machinist) also said not to worry about it: if I'm not vomiting within five minutes of eating out of it, it can't be too poisonous.

Sue