A sharp turn. And this isn't the most mathy definition right over here, but it's easy to recognize. A sharp turn is something like that. Or like, or like, well no, that doesn't look too sharp. Or like this. And the reason why I think where you have these sharp bends or sharp turns as opposed to something that looks more smooth like that, the reason why we're not differentiable there is as we approach this point, as we approach this point from either side, we have different slopes. Notice our slope is positive right over here where as x increases, y is increasing, while our slope is negative here.