Lichens have long served biology classrooms book illustration of mutualism: a fungus supplies structure and moisture while an alga or cyanobacterium contributes sugars through photosynthesis. Recent microscopy, however, complicates this tidy pairing. Many lichens harbor a second, previously overlooked fungal partner, a basidiomycete yeast embedded in the outer layer, whose abundance correlates with visible differences among specimens once classified as a single species. The finding does not overturn the mutualistic model, but it does suggest that what field guides treat as one organism is frequently a small consortium. For students, the practical lesson is modest: the lichen on a stone wall may be less a duet than a quartet, and its outward appearance may reflect which microbes happen to be present rather than the identity of the dominant fungus alone.