Why do leaves change color in the fall?

The longer answer

The green in a leaf is called chlorophyll. A tree uses it like a kitchen: it catches sunlight and turns it into the sugar the tree lives on. Making chlorophyll takes a lot of light and warmth, so when fall days get short and cool, the tree stops making more.

As the green fades, the yellows and oranges that were underneath the whole summer finally get their turn. Some trees, like maples, also make a brand new red coloring in the fall, which is why a single street can glow red, orange and gold at once.

Dropping the leaves is not an accident. A bare tree loses less water and is safer in winter storms and snow. Look closely at a bare twig and you will find tiny bumps: next spring's leaves, already folded up and waiting.

In Britain and many other countries, people say the leaves change colour in autumn. It is the same change, with the same science behind it.

Three things to know

• Trees that keep their leaves all winter are called evergreens. Pine and fir trees have thin, waxy, needle-like leaves that survive the cold, so they stay green all year round.
• Leaves change color because their green disappears first. The yellow and orange were hiding under the green all summer. When the tree stops making green, they finally show.
• A whole forest can turn color in a couple of weeks. Every tree senses the shorter days and cooler air at about the same time, so a hillside can go from green to red and gold almost together.

Try it at home: a leaf color hunt

1. On a walk, collect fallen leaves in as many colors as you can find.
2. At home, line them up from greenest to reddest.
3. Find a leaf that is half green and half yellow. That is the green fading while the yellow shows through.
4. Look at the end of a bare twig for tiny bumps. Those are next spring's leaves, waiting to open.

Press your favorite leaves flat inside a heavy book for a week. They dry flat and keep much of their color.