title: What does << mean in Ruby?
body: <p>I have code:</p>
  
  <pre><code>  def make_all_thumbs(source)
      sizes = ['1000','1100','1200','800','600']
      threads = []
      sizes.each do |s|
        threads &lt;&lt; Thread.new(s) {
          create_thumbnail(source+'.png', source+'-'+s+'.png', s)
        }
      end
    end
  </code></pre>
  
  <p>what does <code>&lt;&lt;</code> mean?</p>