Yeast are small fungi which are incredibly important in the food and beverage industries. Yeast ferement the sugars in fruits to make wine, the sugars in grains to make beers. When grown in the presence of oxygen, yeast give off the gas carbon dioxide which makes bread rise. Yeast can grow with oxygen, (aerobically) or without oxygen (anaerobically.) Because it can grow either aerobically or anaerobically, it is known as a "facultative aerobe."