Yeah yeah, trees are made of wood, but what is wood made out of? What gives them their mass? Is it the energy from the sun, meaning that they can convert energy into mass? Are they made of water or soil? Most people might think that they draw their nutrients out of the soil through their roots, but if this was the case then wouldn't all trees be surrounded by a big ditch after a while when they'd used all the soil up? In the 1600s, a man named Jan Baptist van Helmont performed an experiment in which he grew a willow tree in a carefully measured amount of soil. After five years, he weighed both the tree and the soil and found that the tree was 74 kg, but the soil was only about 60g lighter. This doesn't add up with the laws of the conservation of mass, so the tree's bulk had to be coming from somewhere else. As it turns out, trees are made of air. Or, more specifically, the carbon in the air. We all know that trees "breathe in" carbon dioxide and "breathe out" oxygen. This is handy for us oxygen-breathing life forms, but also means that the trees are doing something important with all that carbon, namely, making wood. When you burn wood, you release that carbon back into the atmosphere where it binds with oxygen to create CO2 all over again.