Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
why do leaves change color in autumn?
All summer, leaves are green because they're packed with chlorophyll, the pigment that grabs sunlight for food.
But chlorophyll is fragile — it breaks down constantly, and the tree keeps rebuilding it using sunlight and warmth.
Autumn brings shorter days and colder nights. The tree senses this and starts sealing off each leaf at its stem.
Once sealed off, the leaf can't make new chlorophyll. The green fades fast, unmasking colors that were there all along.
Yellow and orange come from carotenoids, pigments that were in the leaf the whole time, just hidden under the green.
Red comes differently — some trees make fresh anthocyanin pigment in autumn, often when sunny days follow cold nights.
The green was never the only color — autumn just switches off the mask and lets the real ones shine.