Why do FTS osmotic water filters perform so well in nature? Because they come from nature. Made from plant-derived cellulose, the Rainstick’s membrane filter is tough, loves water, and is virtually clog-proof. This means it works every time, no matter how bad the water source. More importantly, its amazing 0.0003 micron pore size assures safety and peace of mind when you need to drink from a compromised water source. Osmosis is a natural way of filtering water. When placed in a water source, the nutrient powder blend inside the Rainstick draws water through the pouch’s membrane filter walls while blocking contaminants. The result is a clean drink that provides calories and electrolytes.