widget

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Guinness in a can has a widget to help give it a head

Actually that little ball at the bottom of a guiness beer can is called a "Widget".

The cans/bottles are filled under pressure, usually with nitrogen. It is done in such a way that the gas fills the widget through the very small hole, I believe they do it upside down. When the container is opened, the gas escapes through the small hole and creates bubbles in the beer. Without the widget, there wouldn't be any bubbles.

This is what I got off of the guinness.com website:

Glad you asked. The widget is a plastic molded device that sits on the top of the contents of each can of GUINNESS® Draught. When the can is opened, a small amount of beer and nitrogen, trapped in the widget, is forced out through the beer, which creates the famous creamy head that you find on a pint of GUINNESS® Draught served in a pub. The widget gives GUINNESS® Draught in cans the taste and texture of a pub-poured pint at home. In GUINNESS® Draught in bottles, the clever little 'rocket' widget floats free in the beer to refresh the creamy head of your GUINNESS® Draught with each swig you take from the bottle.