Final answer:
A fairy ring on a lawn is composed of mushrooms, specifically from the fruiting bodies of the basidiomycete fungi like Marasmius oreades, forming a circle as the mycelium seeks soil with adequate nitrogen.
Step-by-step explanation:
A "fairy ring" on your lawn is composed of mushrooms. These rings are formed by the fruiting bodies of basidiomycete fungi, which grow underground and expand outward in a circle. As the fungal mycelium grows, it depletes the soil of nitrogen, prompting the mycelium to grow away from the center. This leads to the formation of a ring of mushrooms at the perimeter where there is still adequate nitrogen in the soil. The best-known fairy ring fungus is called Marasmius oreades. Fairy rings are not related to tree rings, which indicate the age and growth conditions of a tree, nor are they related to wedding rings or the rings of Saturn, which are composed of countless icy particles orbiting the planet.