Let's sit out on that curb and measure what this variable is over a bunch of hours and then average it up. And that's going to be a pretty good estimator for the actual mean of our population, or since it's a random variable, the expected value of this random variable. Let's say you do that and you get your best estimate of the expected value of this random variable is, I'll use the letter lambda. So this could be 9 cars per hour. You sat out there, it could be 9.3 cars per hour. You sat out there over hundreds of hours and you just counted the number of cars each hour and you averaged them all up. And you said on average there are 9.3 cars per hour and you feel that's a pretty good estimate.