Suppose someone brings John a glass of tap water, which John watcher being poured from an entirely normal tap. Yet suppose that the water from that particularly tap was somehow laced with poison. When asked what the glass contains, John, not knowing of the poison, says "That's water."
Let's put aside the issue of whether witnessing tap water being poured is sufficient grounds for knowledge that the substance is in fact tap water, and assume that, were the water not poisoned, John would have a justified true belief about the contents of the glass.
Presented with the poisoned water, does John have knowledge about the contents of the glass?
I ask because, normally, our tap water contains a great deal of things besides water, yet we would not intuitively say that calling the stuff that comes from taps "water" is incorrect. But if some of the stuff was poison, it suddenly seems that John's belief that the glass contains water is incorrect (despite, in a sense, being obviously true), because if he were to let that belief inform his actions, the consequences would not be what he expects, to put it lightly. Yet in either case, the water contains a lot of things that are not water; so why is it correct to say "water" when there are lots of non-poisonous contents, but incorrect to say "water" when one of the contents is poisonous?