Made of Air

A big tree weighs more than a car. So where did all that weight come from? Nearly everyone says the soil — that the tree pulled it up out of the ground through its roots. Nearly everyone is wrong. Most of a tree's weight is carbon, and its leaves took that carbon out of the air, as a gas.

This game builds a tree out of the two things it really uses. One button opens the leaves: each press turns a hollow square of carbon dioxide in the sky into a solid square of wood in the tree's crown. The pot lends a square of water for every square of wood, and that water leaves again through the leaves as vapour, so the pot has to be filled up. The block of soil in the pot is drawn at the same size on every single turn, and it never gets smaller.

There are three challenges. In the first you simply grow the tree. In the second the tree is shut under a glass jar, where the air runs out and growing stops — even though the pot below is still full of soil and still holds water. You have to lift the jar to let more air in. The third is a real experiment from about 1648: a gardener weighed a pot of soil, planted a willow in it, and watered it for five years. You guess how much of that soil the pot lost, then live the five years out and see how close you were.