For instance, when we're exercising strenuously the skeletal muscles at some point will not be able to get enough oxygen and at that moment in time they have to switch to the process of lactic acid fermentation because that allows them to continually generate those ATP molecules. And so that's exactly what allows the buildup of lactate. So lactic acid in our body and that eventually causes fatigue. So many prokaryotic and eukaryotic organisms undergo lactic acid fermentation. In fact, when the two supply runs low in our cells they use this process to regenerate the NAD plus. And this process is a single step process that essentially consists of a well, not a single step process but it's more complicated than that.