title_body: What is the science behind the Widget?
upvoted_answer: Widgetized beers are packaged with liquid nitrogen, and the widget, which contains a small hole. The liquid nitrogen warms, turns to a gas inside the sealed can, which forces gas and beer into the widget through the hole. 
  
  When the can is opened, the pressure inside drops suddenly, causing the contents of the widget to be shot into the beer, agitating the beer, producing the foam. 
  
  If this were tried with a regularly carbonated beer, you'd end up with foam erupting out of the can. Hence, beers packaged with a widget have only about 1/3 the carbon dioxide dissolved than regular beers, plus the nitrogen, which creates a smooth mouthfeel and dense foam. Due to the requirements of reduced carbon-dioxide, the widget is used almost exclusively with Stouts, Bitters and Irish Red ales - styles that can be served with lower volumes of carbon-dioxide. 
  
  see
  
  
  http://en.wikipedia.org/wiki/Widget_(beer)#Method