Let's say that you're curious about people's TV watching habits, and in particular, how much TV do people in the country watch? So what you're concerned with, if we imagine the entire country that we've already talked about, especially if we're talking about a country like the United States, but pretty much any country, is a very large population. In the United States, we're talking about on the order of 300 million people. So ideally, if you could somehow magically do it, you would survey or somehow observe all 300 million people and take the mean of how many hours of TV they watch on a given day. And then that will give you the parameter, the population mean. But we've already talked about, in a case like this, that's very impractical. Even if you tried to do it, by the time you did it, your data might be stale because some people might have passed away, other people might have been born.