title: Writing a header in iPython Notebook causes the header to be invisible
body: <p>In <strong>iPython Notebook</strong> running Python 2.x, when we write a markdown <code>#header</code> in a new cell and execute it, nothing appears</p>
  
  <p><a href="https://i.stack.imgur.com/SRhJH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SRhJH.png" alt="enter image description here"></a></p>
  
  <p>Every word I've tried seem to work for the header except <code>header</code>. How can we have a header with the word <code>header</code>?</p>