Another example is the elegant, minute structure of bone tissue. Other holes in the body stimulate tremendous efforts to eliminate them. Dental caries, pitted scars in the skin left behind after smallpox has struck, unsealed passageways in the newborn's heart, and cavities in arthritic bones are all examples of highly undesirable holes. Largely theoretical, holes in solid-state physics refer to the absence of an electron from an atom in certain substances, such as beryllium. Electrical currents move by the motion of these holes.
Some holes are functional, elongated passageways, rather than punctuated voids in solids.
In this group are tubes that transport or allow the passageway of busily moving substances. Short, constructed tubes provide shelter for fragile caddis fly larvae. There are holes that serve as doors or mere openings through which things pass. Stomata are microscopic pores that are part of the epidermal layer of leaves that allow the passage of oxygen, carbon dioxide and water vapor. Millipore filters are precision tools that use the size of the pores or holes to permit certain sizes of substances to go through or be retained.