How do you make code that fits in your head? Carl and Richard talk to Mark Seemann about his latest book that focuses on building sustainable applications. Mark talks about the key issue of any application that is going to exist for a while: Can others understand what you wrote? Making understandable code starts with brevity, but clarity is also important. Modern tooling makes it easier to create sustainable applications, with automated testing and continuous integration and deployment being some of the biggest players. Code that you can learn from is code that fits in your head!