Photosynthesis is pretty amazing when you really think about it (what do you mean, you never think about it?). What photosynthesis describes is how this tiny little seed can grow into a plant as big as a tree. How is this possible?

Think about how animals like us grow. A baby grows in a mother’s womb from a single cell to an 8-ish pound baby. From then on, how does this tiny person grow to be a 150-pound adult? (These weights are just examples. Not every baby and adult have to weigh these amounts. Don’t get hung up on these details.) Where do those other 142 pounds come from? Well, from the food the person eats. Food is energy; food is mass. When a child is growing, the food she eats goes toward her energetic needs–keeping her body temperature up, keeping her muscles moving–and some of it goes towards actual body mass. The atoms you eat (such as carbon, hydrogen, oxygen, nitrogen, etc.) are used to create the mass of your body. That saying, “you are what you eat” isn’t just clever, it’s true. The carbon atoms in the bread a child eats can become the carbon atoms in the cells of her body. Woah. So that means those 142 pounds come from a combination of bread, cookies, broccoli, spinach (yum!), fries, and chocolate shakes–whatever that person ate growing up. Makes you think a little more about your diet, hm?

Since plants don’t eat food, how do they grow? Where does all the “stuff” come from to make a giant tree?

Let’s simplify this: life on earth is carbon-based. All life forms need a source of it. We get carbon from the food we eat–carbohydrate, protein, and lipids (fats) all have carbon. Plants get their carbon from a different source–the carbon dioxide (CO2) in the air! That’s right! There is a great source of carbon floating around in the air all the time. Every time you exhale, you release more CO2 into the air.

Plants use the energy from the sun to change carbon dioxide in the air and water from its roots into sugars that they can use to build the tree trunk, stem, leaves–whatever!–that make up the plant’s body.

This is a very difficult idea for most people to accept. If you give a person a seed and a log and ask him “where did all this mass come from?” he will likely say that it comes from the soil. Maybe you thought that plants grew because they “ate” the soil too. But here is a very easy way to disprove this to yourself: if you plant a seed in a pot, water it, and watch it grow, does the soil start to disappear? No. It stays there. The plant seems to grow from nothing. But what it is actually doing is using air, water, and light to build its own body.

The other hurdle you have to jump to truly understand how this works is the idea that air is nothing. Most people think about air as empty space, but it’s not. Air has mass. Air has stuff. If air moves fast enough, it can knock down buildings. Just ask anyone who has been in a tornado or hurricane. Take your hand and fan your face. Feel all that “stuff” hitting your face. There is definitely something there. Not a ton of mass, but mass nonetheless.

So the next time you see a small flower or a big, shady tree, stop for a moment. Look at it and think about how all that mass–that “stuff”–came from something so simple and seemingly empty as air. Wow.