How do you make yeast naturally?

For bread baking, natural yeast is called Sourdough. You make it as follows: mix half a cup of water and half a cup of flour into a dough. Put the dough in a jar on the counter. Every 24 hours, throw half of it away and add another half-cup of flour and half-cup of water. When it froths up and smells like beer, it's ready; put it in the fridge and use it for sourdough bread.