The cohesion-tension theory states that when water evaporates through the stomata in a plant's leaves, a deficit of water is created. This causes the water molecules just below those that evaporated to move up and take their place. Since water molecules like to stay together, however, the water
molecules just below the ones that moved up also move up, in order to stay close. This causes a chain reaction, eventually causing water from the roots to move up into other parts of the plant.