Hi styley, some drinks are naturaly carbonated, ie during the fermentation proccess, because as sugar is fermented it produces roughly equal ammounts of CO2/ alchohol, for instance a high quality champagne would be fermented till nearly all the sugar had been used but not quite all, it would then be bottled with a small amount of yeast left in it, so that it could finnish it's fermentation proccess inside the stoppered bottle, this creates avery high pressure within, hence the very thick an heavy glass bottles used for this purpose, to finnish the,bottle is inverted for a while to let the yeast settle into the neck of the bottle, it is then removed by a very skillful proccess known as disgorging, the cork is released and replaced in a split second, in which time the yeast is blown out, the cork is then wired on to keep it in place, ok that sounds a long proccess and that's why champers is expensive, if you find a cheap one you can bet the co2 has been added afterwards, hope all this makes sense, and helps, Cheers , pete.