Lichen

The green stuff on this rock is lichen, of which there are many varieties. I can't give you a deep scholarly analysis of lichen, but I remember with perfect clarity the explanation offered by a national park ranger during an evening program at Devil's Tower many years ago on one of our family vacations. Lichen happens, she said, "when Freddie Fungus and Alice Algae take a lichen to each other."

Educators, take note of the reasons that explanation has stuck with me for more than two decades.