RailsCasts (Mobile) is a Ruby on Rails screencast that teaches by building. A typical episode begins with a small application and a clear target. One is a store scaffolded with a product name and price. Another is a blog whose articles each have many comments. A third is a cookbook site where users share recipes, leave comments and add each other as friends. The host then builds a working feature on screen, writing code, reloading the page and checking the result. In '#417 Foundation' that means adding Zurb Foundation to a fresh app, pasting in a prepared top navigation bar, and overriding Sass variables to turn its background dark blue. Along the way the host notes that Foundation is built on Sass rather than Less, which can make it easier to fit into a Rails app, and that it can run on the smaller Zepto.js instead of jQuery. In '#409 Active Model Serializers' the blog gets a JSON API, and its comments move to the root level for the benefit of client-side JavaScript frameworks. Upgrades get the same hands-on treatment. The Rails 4 upgrade walkthrough starts with a checklist: the latest Rails 3.2, up-to-date gems, Ruby 1.9.3 or newer, and a test suite that passes. Then things break. The router rejects the old match method, the specs fail on mass assignment from a Papertrail model, and each problem is fixed until the upgrade goes through. The host does not pretend every fix is elegant. In the activity-feed episode, the model reaches into the controller to record the current user. The host says this workaround feels kind of ugly, then keeps it because it works. Comparisons end with the same honesty: Foundation and Twitter Bootstrap are both called great frameworks, and choosing between serializers and view templates is called a question with no wrong answer. Two episodes open by citing viewer requests. One asked for a GitHub-style activity feed, the other for importing the CSV and Excel files that an earlier episode showed how to export. The series often points back to itself, citing earlier numbered installments on Twitter Bootstrap, Turbolinks and the new features of Rails 4. Episode 400 marks the milestone by trying out Rails 4 from the master branch, celebrating that the public index.html file no longer has to be deleted, and showing how to remove Turbolinks. It closes with a challenge to learn a new language or framework, graphic design, or something outside technology, because trying new things makes for better Rails developers. For a developer choosing a CSS framework, '#417 Foundation' is the obvious entry point, and for one holding a Rails 3.2 app, '#415 Upgrading to Rails 4'.