Why does a raisin, when put in a glass of champagne, float up and down?

When champagne (or soda water) is poured in a glass and raisins are added into it, they sink to the bottom. After some time, the raisins begin to move up and down in the glass as if they are dancing.

This movement is due to the dissolved carbon dioxide in the liquid. The CO2 bubbles cling to the surface of the raisins. Its wrinkles and folds afford a lot of area to the bubbles for attachment. The raisin is lifted by the bubbles to the top. When the bubbles escape into the air, the raisins sink again. The process continues till CO2 is available.