[rthille]

Yeah, a similar observation is that there are more atoms in a glass of water than glasses of water in all the oceans. So, dumping a glass of (somehow trackable) water into the ocean would eventually cause every glass of water to contain atoms from that glass. See also: http://samkean.com/books/caesars-last-breath/

[robocat]

Or it takes 10 years for one of your breaths to be spread around the atmosphere, and everyone is very likely to be breathing at least one molecule of that breath in every breath they take.

From article "Breathing Everyone's Air" - scroll down to that heading in http://www.scifun.ed.ac.uk/card/facts.html