When you pour water into a clean glass in a small confined room with no or little fresh air circulation, the only smells you perceive are those of the glass and the water (or the impurities from both of them).
However, expose that very glass to a bit of fresh air, and voilà, it starts to smell different, a bit like the (wet) shells of mollusks in the sea or from a sweetwater source (it is not a "fishy" smell in the sense that is smells like fish but rather one reminding of the sea, lakes or rivers).
I found this smell with wet glass, ceramics, porcelain, but apparently not really or not at all with plastic or metal (maybe the smell of metal somehow obstructs perception, or maybe it alters or destroys the chemical compounds).
So what is this smell actually? What chemicals are there causing this smell?
Why does it apparently need contact with fresh air (maybe oxygen radicals or something) to emerge? And it seems to be volatile so that the smell weakens with time (or maybe you just get used to it).
I hope at least some here know what I am talking about... if not, maybe try it out.
Some people are apparently more susceptible to this smell than others (a friend of mine is very susceptible to it while others have no idea what this fuss is all about).
It also works when you get the glass hot and fresh from a dishwasher, though it is recommended to try with a non-scented detergent.
And you absolutely have to try it with non-chlorinated water.
Update: I've come across two substances that may be related to the phenomenon, Trimethylamine and Dimethyl sulfide, however, I don't know how they smell, so I can't really say if any of these are responsible for the odors, but from the Wikipedia articles, trimethylamine sounds more fishy than "mollusky" while dimethyl sulfide is described to be cabbage-like.