We all enjoy the beautiful show of color1s as leaves change each autumn. But did you ever wonder how and why this happens? To answer the question, first you have to understand what leaves are and what they do. Leaves are nature's food factories. Plants take water from the ground through their roots. And they take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose  . Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into sugar is called photosynthesis  . That means, "putting together with light". A chemical called chlorophyll   helps photosynthesis. Chlorophyll is what gives plants their green color1. As summer ends and autumn comes, the days get shorter and shorter. This is how trees "know" to begin getting ready for winter. During winter, there is not enough light or water for photosynthesis. Trees rest during this time and live off the food they stored during the summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange leaves. Small amounts of these color1s have been in the leaves all along. We just can't see them in the summer, because they are covered up by the green chlorophyll. The bright reds and yellows we see in leaves are made mostly in autumn. In some trees, like maples, glucose is caught in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn turn this glucose into a red color1. The brown color1 of trees like oaks   is made from waste left in the leaves. It is the mixture of all these things that makes the beautiful color1s we enjoy during autumn. What's the author's purpose of writing the passage?
To explain the process of trees' photosynthesis. To encourage readers to protect the environment. To analyze the reasons why leaves change color1s. To help readers understand what leaves and trees are.