A simple and clever proof of the "Second Partials Test" for optimizing real-valued functions of two variables that does not depend on any fancy mathematics beyond the basic multivariable calculus you've already learned. Need to review single-variable calculus? Why not (re)learn it from 𝘍𝘶𝘭𝘭 𝘍𝘳𝘰𝘯𝘵𝘢𝘭 𝘊𝘢𝘭𝘤𝘶𝘭𝘶𝘴: 𝘈𝘯 𝘐𝘯𝘧𝘪𝘯𝘪𝘵𝘦𝘴𝘪𝘮𝘢𝘭 𝘈𝘱𝘱𝘳𝘰𝘢𝘤𝘩 this time around? Find it at http://bravernewmath.com. All the cool kids are using infinitesimals, which are oh so much sexier than limits.