This is a great little accessory that allows you to store essentials on the top of your Nalgene bottle. You can store vitamins, supplements, medication, or even water purification tablets right on top of your bottle. It has dividers that allow you to use the lid completely open, divided in half, or divided in quarters. The Pillid can also be stacked providing even more versatility. It fits all Nalgene 32 oz wide mouth bottles (it does not fit the Klean Kanteen wide mouth bottle or the GEN3 Pathfinder Stainless Steel Widemouth Bottle). To use, you simply screw the Pillid to the top of your bottle and then use the original cap as a cover for the Pillid. This is really a handy little device!