The round holes seen in some cheese, called eyes, are not cut into the cheese but form naturally as it ripens. Swiss type cheeses are made with a bacterium, Propionibacterium freudenreichii, that consumes lactic acid and releases carbon dioxide gas as a byproduct. This gas slowly builds up at weak points inside the cheese and pushes the curd apart, leaving behind rounded holes. This distinctive feature is what makes Swiss cheese recognisable, so eyes are characteristic of Swiss cheese.