Plants are green because they contain a pigment called chlorophyll. Chlorophyll is responsible for absorbing sunlight and converting it into energy through a process called photosynthesis.

Here's a breakdown:

* Chlorophyll: This pigment is found in organelles called chloroplasts within plant cells. It absorbs light energy, primarily in the red and blue wavelengths of the visible spectrum.

* Photosynthesis: This is the process where plants use the absorbed light energy to convert carbon dioxide and water into glucose (sugar) and oxygen. Glucose provides the plant with energy for growth and other processes.

* Green reflection: Chlorophyll does not absorb the green wavelengths of light very well. Instead, it reflects these wavelengths, which is why we perceive plants as green.

In essence, plants are green because they are efficient at absorbing most colors of light except for green, which they reflect back to our eyes.