@dalehileman,
I just read about this and what I understand is that the ball is full of beer, but has a tiny hole in it. The beer is bottled or canned under pressure with nitrogen. When the can is opened, the pressure inside drops dramatically and the beer that was in the widget is forced through the tiny hole, creating the tiny bubbles that give the BEST BEER IN THE WORLD it's distinctive creamy head.
Disclaimer: I don't actually work for Guinness. (But how cool would it be if I did!)