You know it's fall when the leaves start to turn. But what causes this riot of color?
Deciduous leaves have four pigments year-round: green, yellow, orange and red.
Most of the time, they're green. The color comes from chlorophyll, which the trees use to photosynthesize.
In fall, days are shorter. With less sunlight, chlorophyll becomes less important, and the leaves make less of it. The other colors get their chance to show.
When it happens depends on how cold it gets. Trees will change their colors in cooler weather. When it gets really cold, the leaves just fall off.