"Rigor leads to rigor mortis," says MIT's Sanjoy Mahajan who teaches his students to use common sense and best guessing to arrive at practical solutions problems great and small. "The world is messy, so do the best you can. You know more than you think you do. Use whatever tools are available to do the job. It's as much an attitude as a technique. And it's an attitude that isn't the norm in mathematics classes, where down-to-the-decimal accuracy is prized.