title: Cannot use list() more than once on a map object
body: <p>Does anyone one know why:</p>
  
  <pre><code>x2 = map(lambda x: x, [2, 4, 6])
  print(list(x2)) 
  print(list(x2))
  </code></pre>
  
  <p>gives:</p>
  
  <pre><code>[2, 4, 6]
  []
  </code></pre>
  
  <p>It is as if I can only use <code>list</code> once on the <code>map</code> object. I do not understand what is happening.</p>