 This is the drinking duct demonstration of evaporative cooling. We'll add some liquid water to the head of the duct. As the water evaporates, the high energy molecules escape, leaving the lower energy molecules behind. The surface of the duct's head decreases in temperature. That reduces the pressure inside the duct's head. As the pressure inside the duct's head decreases, the liquid from the body of the duct flows up into the head. Eventually the head becomes heavy enough to tilt the balance. The duct drinks. The liquid then drains out of the head back into the bottom reservoir. The duct rises. The duct will continue to drink and rise as long as there's liquid on the head of the duct.