So for example, this could be an absolute value function. It doesn't have to be an absolute value function, but this could be Y is equal to the absolute value of X minus C. And why is this one not differentiable at C? Well think about what's happening. Think about this expression. Remember, this expression, all it's doing is calculating the slope between the point X comma F of X and the point C comma F of C. So if X is say out here, this is X comma F of X, it's going to be calculated, and so as we take the limit as X approaches C from the left, we'll be looking at this slope. And then as we get closer, we'll be looking at this slope, which is actually going to be the same. In this case it would be a negative one.