Plants are, in large part, literally defined by photosynthesis. It's practically alchemical, that ability to live on just air and water and sunshine. Yes, we know precisely how the process works -- it's a complex chain of chemical reactions whose details I won't discuss here -- but that doesn't remove the wonder. Photosynthesis produces the sugars that fuel a plant's metabolism and make up the building blocks of its structure. It is the source of well over 90% of a plant's mass.
Now consider this: that mass is mostly pulled out of thin air.
Let me explain, and bear with me for a minute. Photosynthesis, to make it simple, takes water from the soil and carbon dioxide from the air, then uses energy from light to pull an oxygen out of them and rearrange their remaining atoms into sugar. The overall chemical equation looks like this:
6CO2 + 6H2O + energy --> C6H12O6 + 6O2
Six molecules of carbon dioxide plus six of water plus energy are rearranged to give one molecule of glucose -- sugar -- and six of oxygen. There are quite a lot of intermediate steps, but this is what you get when you look only at what goes in (CO2, water, sun) and what comes out (sugar, oxygen).
The really fascinating thing, when you think about it, is where each of those atoms comes from and where it goes. All the carbon and oxygen atoms in the sugar come from CO2: the ones from water are thrown away as oxygen. Water only contributes hydrogen, which is important but not very heavy -- about 4% of the sugar's mass. Sunlight is absolutely crucial to the process, but it contributes energy, not matter. In the end, fully 96% of the sugar's mass comes from carbon dioxide.
That's what I mean when I say that most of a plant is made of thin air. Nearly 96% of its mass comes from just that. The rest comes from water, plus a fraction of a percent from trace minerals in the soil. Air, water, and soil, all of it held together with sunlight: it's the stuff of life.
That's not just true of plants, either. In virtually every biome on Earth, plants form the bottom of the food chain. The sugars they make provide energy for them, and for the herbivores that eat them, and for the carnivores that eat the herbivores, and so on. Almost every living thing is ultimately made of plant matter. We are made of plant matter: everything humans eat can be traced back to a plant.
When you get right down to it, then, all life on Earth is made of very simple things. We are air and water and a little bit of soil, all held together with sunlight.