Why do raisins fall to the bottom of cake?

Raisins fall to the bottom of cakes because of the density difference between raisins and cake batter. Raisins are denser than the batter, so they sink to the bottom. This can be prevented by tossing raisins in a little flour before adding them to the batter. The flour will help to keep them suspended in the batter and prevent them from sinking. Alternatively, you can reduce the density of the cake batter by adding more liquid. This will make the batter less likely to sink the raisins.