XIKAR humidifiers are a two-way system, meaning they 1) emit and 2) absorb moisture to maintain 70% relative humidity. Varying humidity levels in a humidor - too much or too little - can destroy cigars quickly, so consistency is most important in preserving and protecting your expensive cigars. All XIKAR humidifiers employ the use of crystals that can absorb 400x their volume (size) of PG solution. When dry, the crystals look like salt grains. When PG solution is added, the crystals saturate and grow to completely fill the humidifier casing. Thus, you can pack a greater volume of solution in your smaller XIKAR humidifier than you can with older, larger foam units (which are also only one-way systems). Use the extra space in your humidor for more cigars!