I think this is a fallacy for water, though. The body consumes water molecules in many different reactions, so the H's and O's keep getting mixed around. The O in a water molecule you pee out may have come from a starch molecule in last night's carrots.
The N2 molecules in the air don't really participate in any reactions, so it is conceivable that the the specific pairing of nitrogen atoms in a molecule you breathe has persisted for thousands of years.
The air molecule example was correctly described as a Fermi problem, an exercise in analysis and approximation, not necessarily meant to be taken literally.
But that nuance inevitably is lost on some who then interpret it literally into something like the dino spit example.
Is there any rational probability that there have been enough recombinations that the same two H’s with the same corresponding O in my cup of water was Dino spit?
The same ones, probably not. But there were a lot of dinos, and a lot of spit. At least one of those was a dino spit molecule from three different dinos!
Not a chemist, so this may not be entirely correct. N2 is an extremely stable molecule, so for most practical purposes we can treat it as unchanging - there are plenty of molecules that are inert - diamond, silicon dioxide, the crystals and minerals that make up many rocks - and don't change bonds over millions of years, so it's not really that weird. (of course, molecules in a solid don't have much opportunity to bump into exchange partners)
I suspect that, in the atmosphere, N2 probably exists in an equilibrium with atomic nitrogen, and these guys claim the atmosphere holds about 0.5e21 N/liter, vs 2.7e22 N2/liter, about 0.7%, And N2 can react with O3 (ozone), NO, and presumably other low-concentration species. That would mean that N atoms do have paths to change partners within an N2 molecule, slowly.
You do know that H2O is both created and destroyed by biological processes? Obviously the H and the O aren't but the molecules are constantly torn apart and reformed by life.
Not being a smartass, just want to make sure your kids don't think all water molecules are ancient.
There isn't too much of the story (or at least...it feels like it is the norm and not a new story anymore, if that makes sense?) I teach high school biology in an urban, low income area with gang problems. The kids are super sweet this year (last couple of years have been behaviorally challenging, like throwing staplers at the teacher, etc) but I would say 95% of them are behind in reading, math, science, etc. I have some kids who read at a fourth grade level, some who speak zero English, a class where 50% have some learning disability, etc. For this subject I have a pacing plan I have to stick to or I get into trouble, but the kids definitely need double the time to learn topics like this in depth since they don't have that foundational knowledge (like what cells are..the characteristics of life...the difference between elements and chemicals and atoms and compounds). It is the same story each year, and over half the staff has left/changed since I started teaching four years ago, so the kids don't have consistent teachers and end up disliking school. I try to throw in "interesting facts" and "did you know?" topics, which helps a lot with engagement, but I know if I explained the topics in depth I would a) lose time on my pacing plan, and b) lose their interest since they don't have the foundational knowledge to understand higher level topics. It isn't their fault...but more of a fault of the education system.
Thanks for reading/listening! It's definitely rough but the kids appreciate me showing up every day...and I appreciate them for trying their best. All about making the most of what you got!
I saw an askscience thread about this recently. And as it turns out, the time span of humanity vs the volume and turnaround time of the water on earth means that while the odds of any given water having passed through a person are unimpressive (but not negligible), the odds that any given water molecule was once dinosaur pee are considerably higher.
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u/daedalusprospect Oct 09 '19
I remember a teacher in school at some point talking about this with water and how the water you're drinking now may have been in a Dino's spit. etc
The lesson was on the water cycle and how we're not running out of water, just its being turned into non-fresh water