title: Kivy error: NameError 'Clock' is not defined
body: <p>I am picking up Kivy and stuck on the titled error. I am simply trying to run a function 1 min after the app started:</p>
  
  <pre><code>import kivy
  kivy.require('1.9.1')
  from kivy.app import App
  
  class ScreamApp(App):
      def on_start(self):
          Clock.schedule_interval(self.wakemeup, 60)
  
      def wakemeup(self, nap):
          #vibrate here
      pass
  
  
  
  if __name__=="__main__":
      ScreamApp().run()
  </code></pre>
  
  <p>When I run it, it says:</p>
  
  <p>NameError: name 'Clock' is not defined. It is a built in class, according to the documentation. What am i mmissing?</p>