title: why fs.createReadStream only pipe one time?
body: <p>I want pipe <code>fs.createReadStream</code> twice, the code is here:</p>
  
  <pre><code>fs.createReadStream('pdf-sample1.pdf')
    .pipe(fs.createWriteStream('pdf-sample2.pdf'))
    .pipe(fs.createWriteStream('pdf-sample3.pdf'))
  </code></pre>
  
  <p>But i meet an error:</p>
  
  <pre><code>Error: Cannot pipe. Not readable.
      at WriteStream.Writable.pipe (_stream_writable.js:162:22)
      at repl:1:86
      at REPLServer.defaultEval (repl.js:132:27)
      at bound (domain.js:254:14)
      at REPLServer.runBound [as eval] (domain.js:267:12)
      at REPLServer.&lt;anonymous&gt; (repl.js:279:12)
      at REPLServer.emit (events.js:107:17)
      at REPLServer.Interface._onLine (readline.js:214:10)
      at REPLServer.Interface._line (readline.js:553:8)
      at REPLServer.Interface._ttyWrite (readline.js:830:14)
  </code></pre>
  
  <p>who can tell me the reason?</p>