What causes mushrooms to grow in your grass?
Mushrooms are fungi. The main fungus is underground, where it works at decomposing dead plant & animal material in the soil. The mushrooms are the reproductive parts of the fungus that pop up out of the ground to spread spores. They like to grow in shady spots with good soil.
Mushrooms grow from spores that are so tiny you can't see individual spores with the naked eye. Because the spores don't contain chlorophyll to begin germinating (as seeds do), they rely on substances such as sawdust, grain, wooden plugs, straw, wood chips, or liquid for nourishment.