title: Rails - Understanding application.js and application.css
body: <p>New to rails. Just trying to understand these two files in the \assests directory.</p>
  
  <p>For example, the application.js file has lines such as:</p>
  
  <pre><code>//= require jquery
  //= require jquery_ujs
  //= require_tree .
  </code></pre>
  
  <p>I understand the <code>require_tree .</code> simply adds all JS files in the current directory. And by context, I can tell that <code>require jquery</code> adds the jQuery libraries. But where does it get these jQuery libraries from? I don't see any jquery.js files in my assets folder -- or in my entire application directly for that matter? </p>
  
  <p>Similarly, I'm following some instructions to install twitter bootstrap (<a href="http://rubydoc.info/gems/bootstrap-sass-rails/2.3.2.0/frames" rel="noreferrer">http://rubydoc.info/gems/bootstrap-sass-rails/2.3.2.0/frames</a>). After adding my gems to the Gemfile, I need to add <code>//= require bootstrap</code> to the application.js file, and <code>@import 'bootstrap'</code> to my application.css and now it magically works!!! Why!? I can't find these files anywhere</p>
  
  <p>Thanks!</p>