Whats the point of those things on top of a soda cup lid that everyone

[Sartartia08l09]

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