title: Why can I call the .between? method on a Fixnum?
body: <p>For example, if I type the following in irb, it returns Fixnum.</p>
  
  <pre><code>20.class
  =&gt; Fixnum
  </code></pre>
  
  <p>Also,</p>
  
  <pre><code>20.between?(10, 30)
  =&gt; true
  </code></pre>
  
  <p>However, when I'm looking at the Ruby documentation, it says that .between? is part of the Comparable Module, which I didn't call yet.</p>