It is important to remember that differential equations have been studied by a great many people over the last 300 years, and during most of that time there were no TVs, no cell phones, and no Internet. Given 300 years with nothing else to do, you might get quite adept at changing variables.
- from Differential Equations by Paul Blanchard, Robert Devaney, and Glen Hall
Santa Fe Institute Short Course on Networks and Big Data. Hands-on activities for Day 1, Day 2, and Day 3.
Math 246: Differential Equations for Scientists and Engineers