That´s a great question. Plants (everything from tiny algae in the oceans to trees) have green parts (or in some cases are completely green). This is because they contain a green pigment called chlorophyll which is necessary for the plants to make food for themselves using light energy and carbon dioxide (a gas in the air) and water. The green pigment is able to harvest the light energy, and use this energy to change carbon dioxide plus water into sugars that both pants and people eat!

That is a very cool question. To understand it we need to know why anything that isn't giving off light has color. Colors that we see are light that is reflected from a surface and not absorbed. That's why black things get hot in the sun faster than white things. White is the reflection of all types of light, while black absorbs all colors. Plants have a color because they also absorb light. Leaves are full of tiny stuff called chlorophyll ("kloro-fill"). These particles absorb most light but reflect green light. That's why plants are green.

P.S. Chlorophyll is really important for photosynthesis, the process by which plants turn sunlight into sugar (energy).

Plants are green because they reflect green light, and suck up red and blue light. The chemical that does this is called chlorophyll.

First of all, not all trees and plants are green in color, but most are! The reason that most of them are green has to do with the fact that chlorophyll molecules are green.

Chlorophyll is the pigment that comprises chloroplasts, which are the structures or "organelles" responsible for converting the sun's energy into useful energy for a plant. And chlorophyll molecules happen to be green because of their molecular structure.