In much of the United States, fall means hillsides of red, orange, and yellow. Texas has fewer of these displays, but they do exist. At Lost Maples State Natural Area in the Hill Country, bigtooth maple trees turn bright red and gold, and visitors drive for hours to see them. The colors seem to appear out of nowhere. In fact, some of them were there all along.

A leaf's main job is to make food for the tree through photosynthesis, a process that uses energy from sunlight. The substance that captures that energy is chlorophyll, and chlorophyll is green. During spring and summer, a leaf holds so much chlorophyll that its green color masks every other color in the leaf.

Those other colors come from substances called carotenoids, which are yellow and orange. Carotenoids are in the leaf all summer, helping to collect light, but they are hidden by the green. Substances of the same kind give carrots their orange color.

As fall arrives, the days get shorter and the nights get cooler. Many trees respond by getting ready for winter. They stop making new chlorophyll, and the chlorophyll already in the leaves breaks down. As the green fades, the yellows and oranges that were underneath it all along finally show.

Reds and purples work differently. They come from substances called anthocyanins, and most trees do not have them in their leaves during the summer. Instead, some trees produce them in the fall, especially when the days are sunny and the nights are cool but not freezing. That is why the brightest red autumns usually follow a stretch of clear, crisp weather.

Eventually the tree seals off the spot where each leaf is attached to its branch, and the leaf falls. By dropping its leaves, the tree saves water and energy through the cold months. In spring, new green leaves will grow, and the cycle will begin again.