For the Times-Georgian Dear Factman, There's this grey, flaky stuff growing on a tree in my front yard. Signed, Allison-Sand Hill Elementary.
Lichens are very interesting things.
They both contribute something to the relationship, and without each other they would die. The algae is a microscopic plant which normally lives in a moist place, otherwise it would dry out. In a lichen, the algae actually lives inside the fungus. Algas can make their own food by using sunlight, air, and water. The fungus can't make its own food, but it can stand a much drier environment.