You can't say, oh, it's a billion, because there's some probability that it might take a billion and one rolls. That is a very, very, very, very, very, very small probability, but there's some probability it could take a Google rolls, a Googleplex rolls, so you can imagine where this is going. So this type of random variable, where it meets a lot of the constraints of a binomial random variable, each trial has a clear success or failure outcome, the probability of success on each trial is constant, the trial results are independent of each other, but we don't have a fixed number of trials. In fact, it's a situation where we're saying, how many trials do we need to get, do we need to have until we get success? Maybe that's a general way of framing this type of random variable. How many trials until success, while the binomial random variable was, how many trials, or how many successes, I should say, how many successes in finite number of trials? So if you see this general form and it meets these conditions, you can feel good it's a binomial random variable, but if you're meeting this condition, clear success or failure outcome, independent trials, constant probability, but we're not talking about the successes in a finite number of trials, we're talking about how many trials until success, then this type of random variable is called a geometric, geometric random variable.