There's a nice Veritasium episode wherein he asks people on the street where a tree's mass comes from: https://www.youtube.com/watch?v=2KZb2_vcNTg (This isn't just one of those "let's laugh at how dumb people on the street are" videos; it's done explicitly as a learning exercise. His PhD looked at how addressing misconceptions and going from there to correct explanations works much better than simply providing explanations: https://www.youtube.com/watch?v=RQaW2bFieo8 )

I thought of it upon seeing the first chart in the BMJ article.)

Let me guess: mostly from water? And the air of course - carbon. Sorry, I won't watch the video, its far too spare a use of time.

He pretty much just says carbon. Certainly is some water involved too, but yeah, breath in CO2 and breath out just O2.

Let Feynman explain it to them - https://www.youtube.com/watch?v=N1pIYI5JQLE