Learned something fun today ... a method for solving a system of linear equations called “Gaussian elimination”. Something I'm working on now requires solving thousands of these things, and I certainly didn't want to do it by hand. The manual method that I knew of didn't lend itself to being implemented in software, but the Gaussian elimination does so admirably. I actually implemented it on a spreadsheet, and it worked beautifully.
It's rare that I implement something in software without understanding how it works – but this will likely be one of them. I've studied this article until my brains hurt, and I still have absolutely zero idea how it works. It might as well be black magic so far as I'm concerned :)