I wouldn't touch that stuff they've been telling you is Guinness, Carole.

But perhaps it's a linguistic confusion as to what "flat" means.

A glass of Guinness is composed of two parts - there's the black bit, which is the liquid, and that doesn't have any bubbles in it by the time you drink it; and there's the white bit at the top, the head, which is all bubbles that have been down the bottom and come up to the top again. And you have to have both bits. A flat Guinness would be one without the head, and it's none too pleasant when you come across it next morninmg in a glass that's been left unfinished at a party.

And all this works better with the draught variety (on draught or in a can with a cunning widget). With the bottles you should still get a head, but the pouring and clearing process is a lot less exciting.