question: How do certain hairs on our body know not to exceed a certain length?
comment: They don't. They're hairs. Hairs don't know anything. They just get produced by special cells in your pores. These cells operate in two cycles. The first cycle lasts a long time. In it it produces more hair, causing the follicle to grow in length. During this time it holds on tight to the follicle as it adds more molecules to the chain. The second cycle is relatively short. The cell stops producing the molecules and releases the follicle from its production line. During this time the follicle is much more likely to fallout or be pulled or rubbed off. The reason hairs only seem to reach certain lengths is because those links are directly proportional to the amount of time those cells spend in their production cycle. EDIT: There's actually 3 phases. Anagen, the growth phase. The Catagen and Telogen phases map to the "Second cycle" i mentioned.
passages: Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen. Each phase has specific characteristics that determine the length of the hair.
   Section::::Description.:Human hair growth.
  
  Hair grows everywhere on the external body except for mucus membranes and glabrous skin, such as that found on the palms of the hands, soles of the feet, and lips.
  
  Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases; all three occur simultaneously - while one strand of hair may be in the anagen phase, another may be in the telogen phase. Each has specific characteristics that determine the length of the hair.
   The argument for this position is complex, and how 'incomparability as vagueness' is to be understood depends on one's theory of vagueness. But the main idea behind the theory is fairly simple. What is the "precise" minimum number of grains of sand needed to count as a heap, or hairs required to count as non-bald? If there is no precise number, only a rough range, then these are instances of vagueness. On one set of theories of vagueness, it is "indeterminate" how many heaps or hairs are required. Perhaps our language simply does not specify a sharp boundary.
   Displacement and vibration of hair shafts are detected by hair follicle nerve receptors and nerve receptors within the skin. Hairs can sense movements of air as well as touch by physical objects and they provide sensory awareness of the presence of ectoparasites. Some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter.
  
  Section::::Functions.:Touch sense.:Eyebrows and eyelashes.
   Hair grows in cycles of various phases: anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase (names derived using the Greek prefixes ana-, kata-, and telos- meaning up, down, and end respectively). Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one or several hairs that might arise from a single follicle exits. Normally up to 90% of the hair follicles are in anagen phase, while 10–14% are in telogen and 1–2% in catagen. The cycle's length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hair have a much shorter length limit compared to hair on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor. DLX3 is a crucial regulator of hair follicle differentiation and cycling. Specifically, colocalization of phosphorylated Smad1/5/8 complex and DLX3 regulate role for BMP signaling to Dlx3 during hair morphogenesis in animal models.
   Any of the various types of hairs may be , producing some kind of secretion, such as the essential oils produced by mints and many other members of the family Lamiaceae.
  
  In describing the surface appearance of plant organs, such as stems and leaves, many terms are used in reference to the presence, form, and appearance of trichomes. The most basic terms used are glabrous—lacking hairs— and pubescent—having hairs. Details are provided by:
  
  BULLET::::- glabrous, glabrate – lacking hairs or trichomes; surface smooth
  
  BULLET::::- hirsute – coarsely hairy
  
  BULLET::::- hispid – having bristly hairs
  
  BULLET::::- articulate – simple pluricellular-uniseriate hairs
   The hair follicle is a dynamic organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between hormones, neuropeptides and immune cells. This complex interaction induces the hair follicle to produce different types of hair as seen on different parts of the body. For example, terminal hairs grow on the scalp and lanugo hairs are seen covering the bodies of fetuses in the uterus and in some new born babies. The process of hair growth occurs in distinct sequential stages. The first stage is called "anagen" and is the active growth phase, "telogen" is the resting stage, "catagen" is the regression of the hair follicle phase, "exogen" is the active shedding of hair phase and lastly "kenogen" is the phase between the empty hair follicle and the growth of new hair.
   Section::::Evolution.:Texture.:The EDAR locus.
   Section::::Hair growth.:Anagen phase.
  
  Anagen is the active growth phase of hair follicles during which the root of the hair is dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2–7 years; this period is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
  
  Section::::Hair growth.:Catagen phase.
   In terms of development, the varying lengths of nectar spurs has been found to be based solely on the anisotropic elongation of cells. However, it still remains to be understood which genes underlie the elongation of cells to form a spur. Are the same genes being co-opted over and over again across the angiosperms to form spurs, or are there several developmental pathways to make a spur?
   These numbers represent the conserved attributes of an object which can be determined from a distance by examining its gravitational and electromagnetic fields. All other variations in the black hole will either escape to infinity or be swallowed up by the black hole.
   Section::::Growth cycle.:Catagen phase.
   There are three major epidermal cell types which all ultimately derive from the outermost (L1) tissue layer of the shoot apical meristem, called protodermal cells: trichomes, pavement cells and guard cells, all of which are arranged in a non-random fashion.
   Fast Adaptation: During fast adaptation, Ca ions that enter a stereocilium through an open MET channel bind rapidly to a site on or near the channel and induce channel closure. When channels close, tension increases in the tip link, pulling the bundle in the opposite direction. Fast adaptation is more prominent in sound and auditory detecting hair cells, rather in vestibular cells.
   Hair plexus
  
  A hair plexus or root hair plexus is a special group of nerve fiber endings and serves as a very sensitive mechanoreceptor for touch sensation. Each hair plexus forms a network around a hair follicle and is a receptor, which means it sends and receives nerve impulses to and from the brain when the hair moves.
   The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year. 
  
  Section::::Growth cycle.:Anagen phase.
   Root hair cells are outgrowths at a tip of the plant's roots. Root hair cells vary between 15 and 17 micrometres in diameter, and 80 to 1,500 micrometres in length. They are found only in the zone of maturation, and not the zone of elongation, possibly because any root hairs that arise are sheared off as the root elongates and moves through the soil. 
  
  Root hairs grow quickly, at least 1μm/min, making them particularly useful for research on cell expansion.
  
  Section::::Importance.
   Hair length is most often measured (in centimeters or inches) from the line of the scalp on the forehead up over the highest point of the skull to its termination, and sometimes from the crown, the latter resulting in 10 centimeter lower values in average. In cosmetology, hair lengths are usually categorized according to the part of the body where the bulk of the longest hair terminates: "chin level", "shoulder length", "lower shoulderblade"/"mid-back level", "waist length", "hip-length", "classic length" (extends to almost "buttock-length", i.e. upper thigh-length, where the legs meet the buttocks), "thigh-length", "knee-length" and "ankle/floor length" hair.
   On April 28, 2004, the SEC voted unanimously to permit the largest broker-dealers (i.e., those with "tentative net capital" of more than $5 billion) to apply for exemptions from this established "haircut" method. Upon receiving SEC approval, those firms were permitted to use mathematical models to compute the haircuts on their securities based on international standards used by commercial banks.
   Slow Adaption: The dominating model suggests that slow adaptation occurs when myosin-1c slides down the stereocilium in response to elevated tension during bundle displacement. The resultant decreased tension in the tip link permits the bundle to move farther in the opposite direction. As tension decreases, channels close, producing the decline in transduction current. Slow adaptation is most prominent in vestibular hair cells that sense spatial movement and less in cochlear hair cells that detect auditory signals.
  
  Section::::Neural connection.
   Section::::Example.
   There is a passing, satirical, allusion in English to this version of the principle in "A History of the Athenian Society", prefixed to ""A Supplement to the Athenian Oracle: Being a Collection of the
  
  Remaining Questions and Answers in the Old Athenian Mercuries"", (Printed for Andrew Bell, London, 1710). It seems that the question "whether there were any two persons in the World that have an equal number of hairs on their head?" had been raised in "The Athenian Mercury" before 1704.
   The genetic control of patterning of trichomes and roots hairs shares similar control mechanisms. Both processes involve a core of related transcription factors that control the initiation and development of the epidermal outgrowth. Activation of genes that encode specific protein transcription factors (named GLABRA1 (GL1), GLABRA3 (GL3) and TRANSPARENT TESTA GLABRA1 (TTG1)) are the major regulators of cell fate to produce trichomes or root hairs. When these genes are activated in a leaf epidermal cell, the formation of a trichrome is initiated within that cell. GL1, GL3. and TTG1 also activate negative regulators, which serve to inhibit trichrome formation in neighboring cells. This system controls the spacing of trichomes on the leaf surface. Once trichome are developed they may divide or branch. In contrast, root hairs only rarely branch. During the formation of trichomes and root hairs, many enzymes are regulated. For example, just prior to the root hair development, there is a point of elevated phosphorylase activity.
   Section::::Mammals.
  
  Detection of hydrodynamic stimuli in mammals typically occurs through use of hairs (vibrissae) or “push-rod” mechanoreceptors, as in platypuses. When hairs are used, they are often in the form of whiskers and contain a follicle-sinus complex (F-SC), making them different from the hairs with which humans are most familiar.
  
  Section::::Mammals.:Pinnipeds.
   Section::::Functions.:Protection.
  
  In some mammals, such as hedgehogs and porcupines, the hairs have been modified into hard spines or quills. These are covered with thick plates of keratin and serve as protection against predators. Thick hair such as that of the lion's mane and grizzly bear's fur do offer some protection from physical damages such as bites and scratches.
  
  Section::::Functions.:Touch sense.
  
presuppositions: Hair is self aware. Hairs on the body know not to exceed a certain length.
corrections: Hair operates in cycles set by the body. Hairs on the body don't know not to exceed a certain length.
raw_presuppositions: Hair is self aware. Hairs on the body know not to exceed a certain length.
raw_corrections: Hair operates in cycles set by the body. Hairs on the body don't know not to exceed a certain length.