Mold grows on bread when the bread comes in contact with dust that is carrying fungi, which are a tiny microorganisms. Spores remain airborne until they land on a piece of food, germinate and grow into a healthy fungus.
Mold is usually fuzzy and blue, black or green in color. It is commonly seen on expired bread. Mold is part of a living group of organisms named fungi. Slime molds and mushrooms belong to the fungi family too. Mold lives off the food its growing on by eating away at it. It commonly grows right through whatever it's eating. Fungi break down what they are eating by digesting the juices they squirt out. They then grow microscopic roots into the food itself.