Why do leaves change color in the fall? Sunlight, along with water and carbon dioxide, is necessary for photosynthesis. That's the process by which plants make their food. In fact, the word photosynthesis means putting together (synthesis) with light (photo). Leaves are the food factories in plants, where photosynthesis takes place. Chlorophyll, a chemical that gives leaves their green color, plays a key role in the process. Photosynthesis also releases oxygen into the atmosphere - a good thing, since that's what people breathe. In fall, when there is less sunlight, photosynthesis slows down and then stops, and plants live off the food they have stored. When the green chlorophyll goes out of the leaves, the leaves take on the color of other chemicals in them - that's how leaves get their beautiful autumn colors.
Source: The World Almanac for Kids 2006