In the wild, cats naturally excrete in soft or sandy soil for easy burial. They use their paws in a backward sweeping motion to cover their feces. To stimulate this instinctive desire, a litter box's bottom is typically filled with or less 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, or owners may sprinkle a thin layer in the bottom of the box, under the cat litter. The litter material also satisfies a cat's instinctive desire to hide their scent by allowing them to bury their waste. 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.