When natural materials such as sawdust are compressed as in the pellet mill, the pressure plus the heat caused by the milling produces a natural substance called lignin. See our WIKI page for more on lignin. This makes the pellets hold together. However, depending on how dry the material is, additional moisture may have to be added as you make pellets. This may be as simple as spray the material with a little water, There are also all natural binders on the market that can be added to the material you're pelletizing.