To get energy, we need to breathe. The inhaled oxygen is then converted into energy in our mitochondria, which we need for our metabolism. With some microorganisms, this is a little different, because they can "breathe" other compounds. This is called fermentation. In this process, energy is generated by the degradation of organic substances without external electron acceptors such as oxygen. Depending on which organic substances are degraded, a distinction is made between different fermentations. If sugar becomes alcohol and carbon dioxide, this is referred to as alcoholic fermentation. It is very important for beer production and for bakery products.
Explanation for children:
To get energy we have to breathe. The inhaled oxygen is then converted over several steps into energy, which we need for our metabolism. Microorganisms must also breathe, but they do not necessarily use oxygen, but other compounds. Breathing without oxygen is called fermentation. The microorganisms get energy by degrading different substances. Depending on which substance they degrade, a distinction is made between different types of fermentation. An important example is lactic acid fermentation: bacteria convert lactose into lactic acid, which turns milk into delicious yoghurt.