In a large mixing bowl, combine the flour and water. Loosely place some plastic wrap on the bowl and let the mixture sit undisturbed at room temperature for 4-5 days (the longer it ferments, the deeper the flavor). (Depending on what kind of flour you're using, you may need to add a little more water if the mixture is becoming dry.) The mixture will be fizzy, the color will be very dark and, depending on the humidity, a layer of yeast will have formed on the top. This is normal. Pour off the yeast/mold and as much of the liquid as possible. A clay-like batter will remain. Give it a good stir.