Once, when I was in middle school biology class, the teacher introduced the concept of a symbiotic relationship and gave us the example of a lichen to demonstrate.  

"A lichen," she opined, "is usually a fungus and a photosynthetic partner that depend on each other for survival.  So it's basically a fungus and an algae that are cohabitating."

That's when some kid in the back of the classroom piped up with, "Oooooo! LIVING IN SIN BABY!"

There has not been one time in the rest of my life on this earth since then when I've encountered a lichen and NOT thought about that moment.