The cap of this water bottle contains a totally sealed solar-cell charging system that powers either a white or red LED lamp for gentle area lighting! Fully charged you get 8-10 hours of light with the white LED and 16-18 with the red LED. Waterproof and rugged. So fill your Light Cap bottle with water or whatever you like clip it to your boat or pack and let it soak up the sun's rays during the day. At night a flip of the selector switch turns the bottle into a great lantern. The lid adds a mere 4.5 oz. to the weight of the water bottle - the weight of three swallows of water or a couple of energy bars. Every once in a while a product comes along that makes us say "Wow why didn't we think of this!" This is one of those products!