So cosine of x, right over here, the slope of the tangent line looks like it is zero. And so its derivative function needs to be zero at that point. So hey, maybe it's sine of x. Let's keep trying this. So over here, cosine of x, it looks like the slope of the tangent line is negative one. And so we would want the derivative to go through that point right over there. All right, this is starting to seem, it doesn't seem like the derivative of cosine of x could be sine of x.