In the bottle, the yeast eats the sugar, and its byproduct is C02 (aka bubbles!). However, over time, the yeast dies and forms the lees, or the dead yeast that looks like sediment on the bottom of the bottle. You never see this in a finished bottle of sparkling wine, because winemakers remove the lees. Here's how: After the bottles are through aging, they must undergo the process of riddling. Several times each day, the bottles are slowly tilted vertically and shaken carefully to encourage the lees down toward the neck of the bottle.
Photo: Anna Monette Roberts