I have recently started a liking for Guinness, although I prefer light beer. I decided to buy some Guinness in a bottle and some in a can but it's not the same as having the real deal right out the tap in a pub. The one in the can comes pretty close to the draft poured from the tap. So, in order to get the head (that's the cream or foam on top of the beer), the brewers have added a plastic widget filled with nitrogen, which is commonly known as the nitro widget. You can read up how it works here.