The BlockerLite Pump Sack not only helps inflate your Therm-a-Rest air pad, but also doubles as a 20 liter dry sack that protects your gear in a backpack, for instance. Thanks to SealLine technology the bag is waterproof and durable. Compatible with Therm-a-Rest pads that have a WingLock or TwinLock valve, not older ones. Check the extra image to identify which one you have. Fill with air, then proceed to push the air from the bag into your pad *Why use a pump, I mean I've got my lungs?* If you keep inflating your pad by blowing it with your mouth, you'll introduce large quantities of moisture to the inside. Do it many times enough and you'll find mold growth, nasty smells and even a build up of water that causes trouble when freezing. By using a pump you avoid this altogether. In the extra image this pump is shown next to a one liter Nalgene bottle, for scale. The bottle is not included.