A mushroom is a highly complex organism. Here are the essentials:
The mycelium here is visible to the naked eye.
This is a close-up of about four millimeters (a quarter inch) of the same mycelium as seen under a microscope. When conditions are right, the mycelium forms a fruitbody, pushing the mushroom up in order to disperse the spores or to wait for you to come along and pick it.
