title: Confused about 'respond_to' vs 'respond_to?'
body: <p>I am learning Rails with railstutorial.org, and I am confused about something:
  in <a href="http://ruby.railstutorial.org/chapters/modeling-and-viewing-users-two#sec:password_migration" rel="noreferrer">this chapter</a> the author tells us to do some testing in the console with the <code>respond_to?</code> method on a User object, and it works ok. But later, when we write the test for the <code>:encrypted_password</code> attribute, he uses <code>respond_to</code>.</p>
  
  <p>Out of curiosity, I tried <code>respond_to</code> in the console, for a User object, and I get an error saying the method doesnt exist. Alas, if I try to write the test using <code>respond_to?</code> instead of <code>respond_to</code>, the test doesnt run. </p>
  
  <p>Could someone explain me the difference, and why does the test only run with <code>respond_to</code>?</p>