Last night, my son refused to drink some water that had been out for a while. He said that it was "old water." I told him there's no such thing as old water, and then drank the top of the water and told him I just "drank the old part." That worked, but it got me thinking: is there such a thing as old water? I mean, doesn't all water just sit there in a reservoir before it comes out of the tap?