1/ A Dutch Scientist, Van Helmont conducted an investigation on this question in the 1600s. The prevailing theory at the time was that plants grew by eating soil. He weighed a willow tree and weighed dry soil. He planted the tree, watered it and then left it for 5 years.
2/ He then re-weighed the tree, which had increased in mass by over 12 stone. He dried the soil and weighed it, showing that the soil was almost the same mass. He concluded that the tree grew by drinking water. (source: BBC BiteSize) Van Helmont was wrong though...
3/ A few hundred years later, we know that most of the mass of a tree/ plant is from the CO2 in the air- that becomes starch/ cellulose through the process of photosynthesis. Yet, this is counterintuitive because of our deep rooted primitive notion that 'gases do not have mass' 
4/ ...even if we know about climate change and 'carbon sequestration' to reduce the amount of atmospheric CO2, we may still not have connected the dots to realize that most of the mass of a tree is from the air...Arriving at such conceptual understanding is difficult because...
5/ a) the curriculum is packed with a lot of content and different ideas & it takes time to absorb these ideas b) textbooks & teachers rarely connect different ideas c) most exams do not test such conceptual understanding. Yet, conceptual understanding is important....
6/ We are trying to help children (Gr 8/9/10) develop such conceptual understanding through our online course 'Connecting the Dots: Key Concepts in Science & Math' bit.ly/ConceptsCourse . The course involves 36 hours of online interaction over 25 sessions ( once a week)..
7/ ...& offline assignments + online doubt clearing sessions. All this will be spread across 5 months from Sep 2020 to Jan 2021. Taking the 20 question 'concept quiz' is a good way to check if this course is for you. Links in the blog post- bit.ly/GWconceptual
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