Why do dried fruits and beans swell when cooked?

Because of osmosis, which is the movement of water from an area with a high concentration of water to an area with a low cincentration of water through a semi-permiable membrane. The dired fruits and beans have a very low concentration of water and the boiling pot has a high concentration of water. So the water molecules will move from the pot through the membrane of the plant (fruit/bean) and into the plant causing it to swell

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