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Why do leaves change color in the fall?

The changing of color in leaves is due to chemical processes that start in summer and play out over a period of months. In most trees, these processes are triggered by the leaves responding to the days becoming shorter — not, as is commonly believed, by a change in temperature. (In fact, many trees begin to change color before the first frost.)

During the spring and summer, leaves create a substance called chlorophyll, which produces the energy needed for growth. Chlorophyll is green, which is what gives leaves their characteristic color. As summer turns into autumn, the leaves respond to the decreasing amount of daylight by switching from the production of chlorophyll to the production of sugars and amino acids — substances that act as "anti-freeze", preventing damage during the cold winter.

Aside from chlorophyll, leaves also produce other chemicals called carotinoids and xanthophylls, which are orange and yellow respectively. However, during the spring and summer, these colors are overshadowed by the green of the chlorophyll. As chlorophyll production decreases, the green goes away, which unmasks the orange and yellow, causing the leaves to change color. Some trees also produce chemicals called anthocyanins, which are red and purple, making the leaves even more colorful.

Around the world, various traditions have developed as to the significance of the changing colors. In America, for example, there is a Native American myth that hunters in the Heavens kill the Great Bear, and its blood, dripping over the forests, is what colors the leaves red. The yellow is caused by fat dripping as the hunters cook the meat.

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