The labor-intensive process whereby wine undergoes a secondary fermentation inside the bottle, creating bubbles. The process begins with the addition of a liqueur de tirage (a wine solution of sugar and yeast) to a bottle of still base wine, triggering a secondary fermentation inside the bottle which produces both carbon dioxide and spent yeast cells, or lees, which are collected in the neck of the bottle during the riddling process. The lees are then disgorged from the bottle, and replaced with a solution of wine and sugar, giving the sparkling wine its sweetness. All Champagne and most high-quality sparkling wine is made by this process. Also known as méthode Champenoise, méthode classique and metodo classico.