<human>:  What is a lichen?
<bot>: A lichen is a symbiotic relationship between a fungus and an alga. The fungus provides the nutrients for the alga, and the alga provides the fungus with a safe place to live. Lichens are found on rocks, trees, and other surfaces. They are often green, brown, or gray in color, and they are often found in areas with a lot of moisture.