All but the simplest applications have menus.  The system automatically
  lays the menus out and provides standard ways for users to access them.  
  In this sense, they are familiar and dependable ways for users to access
  functionality across all applications.  All menus are panels that "float"
  on top of the activity screen and are smaller than full screen, so that the
  application is still visible around its edges.  This is a visual reminder 
  that a menu is an intermediary operation that disappears