Three and a half centuries ago, a five-year experiment was conducted involving the growth of a willow tree in a pot which received only water. The conclusion, that a tree is therefore made solely from water, was not so ridiculous when there was still general acceptance of the Aristotelian view of only four "elements": water, earth, fire and air. The error of the conclusion leads students straight into a discussion of carbon dioxide as the real basis of the tree's growth via photosynthesis and consideration of the carbon cycle. (Contains 1 table.)