There are some things that EVAPORATE. If you were to put something in the glass and let it sit, perhaps the poison would mix with air and evaporate out of the glass. Chlorine does this from tap water, which is why water out of the tap might smell and taste like chlorine but if you let it sit a bit first, it won't.

As for the second riddle, not only does "time" fit, but so does "water." Think about it.

Oh, and the third riddle...I have no idea.