And if you think about it in terms of derivatives, it means that your first derivative is increasing over that interval, and in order for your first derivative to be increasing over that interval, your second derivative, f prime prime of x, actually let me write it as g, because we're using g in this example, in order for your first derivative to be increasing, your, let me write this, so g, so concave upward means that your first derivative increasing, increasing, which means, which means that your second derivative is greater than zero. And concave downward is the opposite. Concave downward, downward, is an interval, or you're going to be concave downward over an interval when your slope is decreasing. So g prime of x is decreasing, or we can say that our second derivative, our second derivative is less than zero. It's less than zero. And once again, I could draw it on this. So when x is lower, we have a, look, we have a, or it looks like we have a positive slope, then it becomes less positive, and then it becomes less positive, it's approaching zero, it becomes zero, then it becomes negative, and then even more negative, and then even more negative.