Liquer de tirage

Liquer de tirage – French term, used with Champaign
A mix of wine, sugar and yeast is added to a quiet wine.
A quite wine is an already fermented, non-sparkling wine.
The bottle is closed off again and this mix incites a second fermentation on the bottle.
 

During this second fermentation carbon dioxide comes into existence, which cannot escape: the sparkling wine is born.