Scientists know why leaves turn yellow in the fall: Chlorophyll breaks down, revealing the yellow pigment that was there all along. But red? Red is a different story altogether. Leaves have to make a new pigment to turn red. Why would a dying leaf do that? Scientists don't really know. NPR science correspondent Nell Greenfieldboyce reports on the leading hypotheses out there.

Read more of Nell's reporting on this topic .

Interested in more seasonal science? Email us your ideas at shortwave@npr.org .

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This episode was produced by Berly McCoy. It was edited by Reb

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