You can get mold to grow on bread fast when the right conditions are present. Tiny mold spores are in the air we breathe and the dust that tends to collect on every horizontal surface. The porous texture of a bread slice provides plenty of surface area and airflow for mold spores to collect and flourish. You can speed up mold growth even more by manually introducing spores and ensuring sufficiently moist and warm conditions.
Purchase a loaf of preservative-free whole grain bread. Commercially made bread has chemical preservatives that stunt mold growth. Health and natural food stores often carry breads made with all natural ingredients and no preservatives.
Cut a 3/4-inch slice from the center of the loaf. Pull out a slice from the center if the loaf came pre-sliced.
Rub one side of the bread slice on the moldy section of a piece of food or on a dusty ledge. Adjust the nozzle on the spray bottle to a fine mist. Spray the contaminated side of the bread slice a couple times to make it slightly moist.
Slide the moist slice of bread into a zip-top plastic bag and seal the bag.
Place the bag in a warm and dark place with the contaminated side facing up. Check on the bag after one day to see if any small areas of growth have begun. After two or three days, the mold will take over the surface of the bread slice.