title: Split arrays in an array
body: <p>I want to split arrays in an array and get the string of the second elements mixed.</p>
  
  <pre><code>[["c","a","t"], ["d","o","g"]]
  ...
  # =&gt; "ao"
  </code></pre>
  
  <p>How can I do it? I tried the <code>join</code> method and got the second and the fifth ones. But when the word length is different like this:</p>
  
  <pre><code>[["b","i","r","d"], ["c","a","t"]]
  </code></pre>
  
  <p>I don't know how to do it. Could you give me some advice?</p>