If we derive it (on your radio) we get another formula that is precisely that of the surface of a sphere, it is as if measuring volumes now we proceed to measure areas. But there are even more, if that formula of the surface of a sphere, return to derive (in terms of its RADIUS) what you get is the parameter of one of the infinite rings which comprised the area, thus having a jump from the 2 nd dimension that is area until a first dimension which is length. What will happen if resulted it another view (on your radio)? If a field is decomposed into layers (like an onion), and those very thin layers of onion are decomposed into rings, that decompose those hoops? As in points, we have the number of elements that compose this circumference. In the previous example appears the idea derivative with respect to your radio and it is that there has to be some variable allowing to be derived to derive, there has to be a value that you can change because without the not there are bypass. The theme of the field speaks of the derivative with respect to its RADIUS, because that radio is the attribute that determines precisely the size of the field, but when we speak of an irregular cube (from a shoe box) would have to derive with respect to its three variables, height, width, and depth or in other words, X, and y Z. We derivariamos in a function of one or another variable to find the surfaces that would be in an imaginary and very fine (infinitesimal, the smallest possible) cuts of a box, vertical horizontally or profanity. I hope I have been helpful, greetings and wish him the best. Original author and source of the article.