Under the force of gravity, the raisin descends to the bottom. Then gas bubbles form on the rough surface of the raisin.
When there are enough bubbles, the expulsive force (Archimedes' principle) will cause the raisin to rise to the surface of the carbonated water. At the surface, the gas bubbles burst, and gravity causes the raisin to sink back down again.
When there are enough bubbles, the expulsive force (Archimedes' principle) will cause the raisin to rise to the surface of the carbonated water. At the surface, the gas bubbles burst, and gravity causes the raisin to sink back down again.