title: What is expansion and how do we use it?
body: <p>A friend from college is studing web programming using the tapestry framework, and he asked me if could i help him with his homework. 
  One of the questions in the homework says:</p>
  
  <blockquote>
    <p>What is expansion, and how do we use
    it?</p>
  </blockquote>
  
  <p>Its the first time hear about it. </p>
  
  <p>Do you have any idea what is that topic about?</p>
  
  <p>I am kind of confused. Is there any OOP principle with that name?</p>