title: Why do we have to define variables while naming the GraphQL queries?
body: <p>Simply, why is this correct:</p>
  
  <pre><code>query HeroNameAndFriends($episode: Episode)
  {
    hero(episode: $episode)
     {
      name
      friends 
       {
        name
       }
     }
  }
  </code></pre>
  
  <p>and this is incorrect (variable "$episode" is not defined.)?</p>
  
  <pre><code>{
      hero(episode: $episode) 
        {
          name
          friends
          {
            name
          }
        }
  }
  </code></pre>
  
  <p>What's the reason behind this specification?</p>