title: What is an optional value in Swift?
body: <p>From <a href="https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/GuidedTour.html#//apple_ref/doc/uid/TP40014097-CH2">Apple's documentation</a>:</p>
  
  <blockquote>
    <p>You can use <code>if</code> and <code>let</code> together to work with values that might be missing. These values are represented as optionals. An optional value either contains a value or contains <code>nil</code> to indicate that the value is missing. Write a question mark (<code>?</code>) after the type of a value to mark the value as optional.</p>
  </blockquote>
  
  <p>Why would you want to use an optional value?</p>