Do you want to know how and why a fall leaf changes color? We first have to understand what leaves are and what they do.
Leaves are the world's food factories. Plants take water from the ground and take CO from the air. Plants use sunlight to change water and CO into glucose(葡萄糖), a kind of sugar. Plants use it as food for energy and growing.
A chemical called "chlorophyll" in the leaves gives plants their green color. As summer ends and autumn comes, the days get shorter and shorter, and trees "know" to begin getting ready for winter.
During winter, there is not enough sunlight or water. We begin to see yellow and orange leaves. Trees rest during this time and live on the food they store during the summer. They begin to shut down their food-making factories.
The bright reds and purples we see in leaves are made mostly in autumn. In some trees, like maples(枫树), sunlight and the cool nights of autumn turn the glucose(葡萄糖) in the leaves into a red color. And we can enjoy the beautiful red leaves during autumn.