Why do champagne bubbles rise from the bottom of a glass?

The bubbles are filled with carbon dioxide  (CO2), a gas 800 times less dense than surrounding liquid.  Molecules of this gas accumulating in imperfections in the glass and start to form a bubble, whose low density supplies enough buoyancy to break off and float towards the surface. In the process they run into more molecules, making the bubble even bigger and more buoyant, and accelerating its ascent.