Cats bury their urine and feces. Indoor cats are usually provided with a box containing litter, generally consisting of bentonite, but sometimes other absorbent material such as shredded paper or wood chips, or sometimes sand or similar material can be used. It should be cleaned daily and changed often, depending on the number of cats in a household and the type of litter; if it is not kept clean, a cat may be fastidious enough to find other locations in the house for urination or defecation. This may also happen for other reasons; for instance, if a cat becomes constipated and defecation is uncomfortable, it may associate the discomfort with the litter box and avoid it in favor of another location. Daily attention to the litter box also serves as a monitor of the cat's health. Bentonite or clumping litter is a variation which absorbs urine into clumps which can be sifted out along with feces, and thus stays cleaner longer with regular sifting, but has sometimes been reported to cause health problems in some cats.[45] Some cats can be trained to use the human toilet, eliminating the litter box and its attendant expense, unpleasant odor, and the need to use landfill space for disposal. Training may involve four to six weeks of incremental moves, such as moving and elevating the litter box until it is near the toilet, as well as employing an adapter such as a bowl or small box to suspend the litter above the toilet bowl.[46] When training is complete, the cat uses the toilet by squatting on the toilet seat over the bowl. Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium (K), sodium (Na), calcium (Ca), and aluminium (Al). Experts debate a number of nomenclatorial problems with the classification of bentonite clays. Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tons ein, has been used to describe clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite. In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. Other common clay species, and sometimes dominant, are montmorillonite and kaolinite. Kaolinite-dominated clays are commonly referred to as tonsteins and are typically associated with coal. A litter box, sometimes called a sandbox, litter tray, litter pan, or catbox, is an indoor feces and urine disposal box for cats (as well as rabbits, ferrets, small dogs such as Beagles and Chihuahuas, and other pets that naturally or through training will make use of such a repository) that are permitted free roam of a home but who cannot or do not always go outside to relieve themselves. Many owners of these animals prefer not to let them roam outside for fear that they might succumb to the elements or get hit by a car (Tufts Veterinary School, among others, report indoor cats live, on average, ten years longer than outdoor cats[1]). A litter box makes it possible to shelter pets from these risks. In the wild, cats naturally excrete in soft or sandy soil for easy burial. They use their paws to cover up in a back sweeping motion to cover their feces. To stimulate this instinctive desire, a litter box's bottom is filled typically with an inch (2.5 cm) or more of cat litter. Litter box filler is a loose, granular material that absorbs moisture and odors such as ammonia. Some litter brands contain baking soda to absorb such odors. The litter material also satisfies a cat's instinctive desire to use an easily-dug material. The most common material is clay, although recycled paper "pellets" and silica-based "crystal" variants are also used. Sometimes, when an owner wishes to stimulate the cat's natural instincts, natural dirt is used. The environmental impact of using commercial cat litter is a matter of increasing concern