Cup Lid

Whats the point of those things on top of a soda cup lid that everyone pushes down? They’re like bubble shaped?
You know what im talking about? Not a can, but a cup. Like what you get at the movies and stuff.
See the little bubbles? One says OTHER next to it.
http://farm1.static.flickr.com/67/182305615_9853179fc8.jpg?v=0
well now adays at restruants and stuff, you get your own drinks from a soda fountain, or someone brings you your drink in a glass.
Those are there to mark one type of soda from another when a group orders. The person behind the counter remembers which is which.
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