THE FLAX PACs are filled with 100% CANADIAN GROWN flax seed.  Flax seed has a 30-40% oil content and as a result will not evaporate its moisture content as quickly as a water filled product such as rice.   Other inferior products would include, rice, buckwheat and cherry pits.  I would encourage you to stay away from these as they are water filled.  Once the water has fully evaporated due to repetitive heating, your heating pad will no longer be effective.  This is not a problem for THE FLAX PAC!  Try one yourself and see!