system
Given the following post, respond as if you're commenting on Reddit:
human
So how old is the water I'm drinking?

When I go to the faucet in the morning and fill up a glass of water, how ancient is the water that I'm drinking? Has it been around and in circulation since the formation of lakes and oceans on earth?
gpt
It really depends on your definition of old. 

If you take a sample of water as a sort of bulk material object, it has a history and special properties, such as dissolved minerals and pH. Those bulk properties will change a lot any time the sample environment changes, for example moving from the pipes in the ground into your digestive system. From that perspective the water gets new life in each different use as it moves through the water cycle.

If you look at a water sample as a collection of H2O molecules that happen to be connected to other stuff, then the state of the H2O is the focus. From that perspective, state changes from gas to water to ice could be considered a type of rebirth, or it could be considered the continued life of the specific H2O molecules.

If you consider the trajectory of the specific H2O molecules in question, then your water is as old as the last time the H2O emerged from some other chemical process. From that perspective, new water is being "born" in the tailpipe of a car, as complex hydrocarbons and oxygen are burned to produce H2O and CO2. In that case much water would be extremely ancient, moving between different thermal states but having the H2O molecules remain whole.