Almost all code in scheme uses recursion. Andy Balaam explains how some simple recursive functions work, by calling the same function from inside itself. If you're new to Scheme, try http://youtu.be/byofGyW2L10 first. The slides for this video are here: http://www.artificialworlds.net/blog/2012/06/16/scheme-4-recursion-how-to-write-functions-in-scheme/