Leaves turn yellow in the fall because Chlorophyll breaks down, revealing the yellow pigment that was there all along. But why red? Red is a different story altogether. Leaves have to make a new pigment to turn red. Why would a dying leaf go an extra mile to 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. 

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