Because fungi basically destroy stuff, we might consider them bad. They create mold on our food, they spoil meat, and they can cause diseases on plants and animals. For this reason fungi usually have a bad name. But actually, there are many beneficial strains of fungi. Yeast, a type of fungus, can be used to make bread rise and the medicine penicillin, is made from a microscopic fungus.
All fungi begin to grow as a single spore.
There are millions of spores all around us right now. They are floating in the air, in the water, and lying on our furniture. They are just waiting for the right conditions of moisture and food to begin growing. This network of mycelium absorbs nutrients from the food, sort of like how the root of a tree would get water from the soil. As a fungus gets bigger, it spreads outward making a larger and larger spot.