Some rocks are green because they have stuff called lichen growing on them. Lichen is green like a plant but grows and sometimes looks like fungus or mushrooms. This is because lichen is actually a plant called algae and a fungus living together. The fungus gives the algae a safe place to live and water and nutrients for it to grow. The algae gives the fungus sugars to eat when it photosynthesizes. Photosynthesis is how plants make food from the sun. The lichen on the rocks is doing an important job, it is very very slowly turning that rock into soil.