And the quantity of oxygen that we have to use to transform the lactic acid back into Pyruvate is known as the oxygen debt. So we see that the process of fermentation is the method by which we basically regenerate our NAD plus molecules from the NADH molecules, because we need our NAD plus molecules to basically undergo the process of glycolysis and produce our continual source of ATP. And our fermentation only takes place under anaerobic conditions. Remember, under aerobic conditions, we have our citric acid cycle and we have the electron transport chain that actually produces the majority of the ATP molecules, as well as oxidizes the NADH back into the NAD plus. But under anaerobic conditions, we have glycolysis being the only method by which we produce our ATP molecules. And to regenerate those NAD plus, we have to undergo the process of fermentation.