<p>haha, that’s wonderful. :D</p>
<p>I assume you realize that a derivative takes ‘1’ to zero, but the derivative of e^x is just e^x (the derivative of e^x is written as d/dx (e^x)). The derivative with respect to a different variable (in this case, y), however, takes e^x as a constant, so it’s derivative is zero.</p>
<p>Hopefully that makes some sense.</p>