A porous material is a material that contains pores or voids. The skeletal portion of the material is often called the "matrix" or "frame". Porous materials can be characterized by their porosity, which is the total volume of empty or pore space in the material. Porous materials can be natural, such as rocks, soil, and biological tissues, or man-made, such as cements and ceramics. Porous materials are used in many areas of applied science and engineering, including filtration, mechanics, engineering, geosciences, biology and biophysics, and material science. Porous materials can be ordered (crystalline) solids, but they can also be topologically disordered, which can create alternative properties in porous materials. Knowing whether a material is porous or non-porous is important because non-porous materials stand up better to cleaning, whereas porous items tend to hold germs and dirt longer and require laundering.