Unlike humans, plants have to make their own food. They do this by turning water and carbon dioxide into nutrients using sunlight and a chemical known as chlorophyll. Scientists call this process photosynthesis. It's the chlorophyll that gives leaves their green color.
In the fall, many plants stop making food causing the chlorophyll to go away. As the chlorophyll leaves, the orange and yellow colors appear. These colors were in the leaves all summer, but the green chlorophyll covered them up.
What about those red and brown leaves? The red color is caused by food trapped in the leaves while the brown color shows that waste has been left behind.
To learn more about leaves and plants, visit these sites:
What is Photosynthesis?
These links from the University of Arizona offer resources for all grades--from kids to scientists--who want to learn more about how plants make their food.