Chlorophyll
Chlorophyll is the green pigment in leaves that captures sunlight to make food for plants.
Think of it this way
A green solar panel on a roof takes in sunlight and turns it into power for a house. Chlorophyll works the same way inside a leaf. It is the green part that soaks up light from the sun. That light energy is then used to make food for the plant.
Explaining chlorophyll by grade level
Leaves are green because of chlorophyll. It soaks up sunlight. Plants use that light to make their own food. Without it, leaves turn other colors.