A leaf is the flat, usually green part of a plant that grows from its stem or branches. Leaves are like tiny factories where plants make their own food using sunlight, water, and air in a process called photosynthesis. Most leaves are green because they contain chlorophyll, a substance that captures sunlight and gives leaves their color.

Leaves come in countless shapes and sizes. Oak leaves have wavy edges and pointed lobes. Maple leaves look like outstretched hands with five fingers. Pine needles are actually leaves too, just long and thin. Some leaves are as small as your pinky fingernail, while tropical plants can have leaves bigger than a dinner table.

In autumn, many trees stop making chlorophyll, revealing other colors that were hidden underneath: brilliant reds, oranges, and yellows. These leaves eventually fall off, which is why we call this season fall. When trees lose their leaves each year, we call them deciduous.

The word leaf also describes a single sheet of paper in a book. Each leaf has two sides, called pages. You might also hear someone say they're “turning over a new leaf,” which means making a fresh start or trying to do better, like the way spring brings new leaves after winter.