Half a glass of water left on a windowsill was empty after two days, though nobody drank it

Why

Reshma is correct that the water left the glass, and correct that nobody drank it — she has the facts straight; the question is what 'finished' means. Water particles at the surface are always moving, and the fastest of them break free and drift off into the room as vapour, one at a time, with no bubbling and nothing to see. They are still water and still weigh what they did; they are simply spread thinly through the air. Test: put a cold steel glass in that room and watch drops gather on its outside — that is the same water condensing back out of the air, and nobody poured it there. Jatin is right: the water changed state and moved, it did not cease to exist, which is exactly why the sea does not empty and the rains return.