Do you know what you are drinking?

I like this argument put forward by Richard Dawkins:

The biologist Lewis Wolpert calculates 'that there are many more molecules in a glass of water than there are glasses of water in the sea'. Since all water on the planet cycles through the sea, it would seem to follow that every time you drink a glass of water, the odds are good that something of what you are drinking has passed through the bladder of Oliver Cromwell.