An object sinks when it is denser than the liquid around it. Raisins are denser than soda water, so they sink at first. But carbon dioxide — the gas that makes soda fizzy — collects on each raisin's bumpy, wrinkled surface. As enough tiny bubbles cling to the ridges, the raisin-plus-bubbles combination becomes less dense than the water and floats to the top. There, the bubbles pop and release their gas. Without that lift, the raisin sinks again, and the cycle repeats.

Raisins are heavier than water, so they sink when you drop them into a glass of soda water. Then something changes. Carbon dioxide — the gas that makes soda fizzy — collects on the bumpy surface of each raisin. Tiny bubbles cling to the wrinkles and ridges, and once enough gather, that upward push becomes strong enough to carry the raisin to the surface. At the top, the bubbles pop and release their gas. Without them, the raisin sinks again and the cycle repeats.