Sometimes you do know. We seldom know the true standard deviation. So if we don't know that, the best thing we can put in there is our sample standard deviation. So the best thing we can put in there is our sample standard deviation. And this right here, this is the whole reason why we don't say that this is just a 95 probability interval. This is the whole reason why we call it a confidence interval, because we're making some assumptions here. This thing is going to change from sample to sample.