Anhydrous calcium chloride is a hygroscopic substance that is used as a desiccant. Hydrated silica can be dehydrated to produce silica gel, which is used as a desiccant. There are two types of dehumidifiers - condensate dehumidifiers and desiccant dehumidifiers. Bentonite can also be used as a desiccant due to its adsorption properties. Because the anhydrous salt is hygroscopic, it is used as a desiccant. Its extreme hygroscopic character makes LiBr useful as a desiccant in certain air conditioning systems. Owing to its hygroscopic nature, aerogel feels dry and acts as a strong desiccant. Like silica gel, it serves as a desiccant. A variation on the single canister desiccant dryer is the Toilet Paper filter. Although THF is traditionally dried by distillation from an aggressive desiccant, molecular sieves are superior. It is used as a desiccant for drying air streams. Rarely, a fecalith will form around a hairball (Trichobezoar), or other hygroscopic or desiccant nucleus. Preheating of the air is shown to greatly enhance desiccant regeneration. CaH is thus used as a drying agent, i.e. a desiccant. Used as a desiccant, it works by a process called "adsorption".