When the mycelium has eaten enough and the surroundings change, cooler air, more fresh air, more moisture and some light, it forms tiny knots called pinheads. Pinheads grow into mushrooms. The mushroom's only job is to release spores, the fungus's dust-like seeds, from the gills under the cap.
Learn the parts: the cap on top, the gills beneath it, the stem that lifts it. Button mushrooms have a veil, a skin that covers the gills while the cap is closed. Paddy straw mushrooms sit in a cup at the base called the volva.