title: What is `hash` in ruby?
body: <p>Because I forgot an assignment, I read the undefined local variable <code>hash</code> before writing it. Surprise: Instead of getting a NameError, the value was read just fine: It was some FixNum and the program crashed much later.</p>
  
  <p>Investigating on the problem, I did the following:</p>
  
  <ul>
  <li>Open up irb</li>
  <li>type <code>hash</code> and press Enter</li>
  <li>Surprise! the answer is -1831075300640432498 (and surprisingly not NameError, nor 42)</li>
  </ul>
  
  <p>Why is that? Is it a bug or a feature? What am I reading here?</p>