How come a raisin placed in a cup of pure water overnight will puff up with water, in terms of osmosis?
Surely you must have eaten fresh grapes, both white and dark?
Raisins and sultanas (and currants) derive from grapes, which when fresh are full to bursting with water. Of course grapes are so full of water that when you crush them (and ferment them) you can make barrels and barrels of wines.
During drying (usually in sunlight) the fresh fruit releases lots of water so that the grapes become shrivelled and wrinkled. Now dried grapes are sweet, they have a high sugar concentration. When you place them in water they have a higher osmotic potential and the water reconstitutes them. Note that when you cook with raisins and sultanas, say in a pudding or a custard, the dried fruit becomes plump because it soaks up the liquid,