question: Why do zippers work only when grabbed by the dangling part?
comment: There's a little catch in there to stop the zipper from opening of its own accord. If you look closely you might be able to see it
passages: The zipper handle can have a short protruding pin stamped into it, which inserts between the zipper teeth through a hole on the slider, when the handle is folded down flat against the zipper teeth. This appears on some brands of trousers. The handle of the fly zipper is folded flat against the teeth when it is not in use, and the handle is held down by both slider hinge tension and the fabric flap over the fly.
   These zippers are very common where airtight or watertight seals are needed, such as on scuba diving dry suits, ocean survival suits, and hazmat suits.
   Section::::Anti-slide zipper locks.
  
  Some zippers include a designed ability for the slider to hold in a steady open or closed position, resisting forces that would try to move the slider and open the zipper unexpectedly. There are two common ways this is accomplished:
   Section::::Description.
   Metal zippers are most effective when used on heavy weight or thick materials such as denim and leather among other heavy-duty fabrics. These zippers are commonly found in denim trousers and jackets, leather outfits, shoes and heavy luggage and bags. Metal zippers are present in a wide variety of designs depending on their desired use, including open-ended, two way open-ended and two way closed-ended designs among others. These designs can be achieved by sewing one end of the zipper, sewing both of its ends together or allowing both of its ends to fall apart respectively.
   Section::::Description.
   A "zipper blocker" prevents the zipper from sliding down its shaft. One version is a locking safety pin in which a piece of the metal bar simply rests across the shaft, thereby blocking movement of the zipper's channel in that direction. Other versions, which are sewn onto the garment, involve two pieces of metal that hook together, and likewise block the zipper's movement.
  
  Other versions have been created that hold the pull tab on the zipper in place.
  
  In many care facilities, common safety pins are used rather than special purchases of these devices being made.
  
  Section::::Mousetraps.:Types of devices.:Thigh rings.
   The zipper is analogous in function to a drawstring, but different in mechanism. A draw string works by tension in the string drawing together the eyelets of the piece together because the tension acts to straighten the string and so forces the eyelets toward a line. The zipper works by an elastic, that is, reversible, deformation of the "locking members" (teeth). The zipper teeth are shaped and sized so that the forces which act on the zipper when the garment it is sewn on is worn cannot unlock the teeth. The slider constrains the teeth positions, moves them along a given path, and acts on the teeth one-by-one in its "Y-shaped channel" and so can reversibly lock and unlock them. This is a lock and key design. In Sundback's invention the teeth are symmetric with "exterior and interior rounded surfaces" that are "elongated transversely". The teeth have a material part ("external projection") and a space ("internal recess"). The material part of one tooth is slightly smaller than the space on the other and so shaped to act as a "contractible jaw"--the jaw is elastically opened and then closed as it goes over the other tooth. The "snug fit" that results when "one member nests within the recess of an adjoining member" is a stable locked state. The maximum force when the slider operates is in between the unlocked and locked positions, giving two stable mechanical equilibria. The "snug fit" is stable not only to forces from wear that act in the same direction as those of the slider but to transverse and longitudinal (both perpendicular) forces. 
   Forty-two years later, in 1893 Whitcomb Judson, who invented a pneumatic street railway, marketed a "Clasp Locker". The device served as a (more complicated) hook-and-eye shoe fastener. With the support of businessman Colonel Lewis Walker, Judson launched the Universal Fastener Company to manufacture the new device. The clasp locker had its public debut at the 1893 Chicago World's Fair and met with little commercial success. Judson is sometimes given credit as the inventor of the zipper, but he never made a practical device.
   A less common water-resistant zipper is similar in construction to a standard toothed zipper, but includes a molded plastic ridge seal similar to the mating surfaces on a ziploc bag. Such a zipper is easier to open and close than a clipped version, and the slider has a gap above the zipper teeth for separating the ridge seal. This seal is structurally weak against internal pressure, and can be separated by pressure within the sealed container pushing outward on the ridges, which simply flex and spread apart, potentially allowing air or liquid entry through the spread-open ridges. Ridge-sealed zippers are sometimes used on lower-cost surface dry suits.
   The slider can also have a two-piece hinge assembly attaching the handle to the slider, with the base of the hinge under spring tension and with protruding pins on the bottom that insert between the zipper teeth. To move the zipper, the handle is pulled outward against spring tension, lifting the pins out from between the teeth as the slider moves. When the handle is released the pins automatically engage between the zipper teeth again. They are called "auto-lock sliders".
   Columnist Sylvia Porter made the first mention of the product in her column "Your Money's Worth" of August 25, 1958, writing, "It is with understandable enthusiasm that I give you today an exclusive report on this news: A 'zipperless zipper' has been invented – finally. The new fastening device is in many ways potentially more revolutionary than was the zipper a quarter-century ago." 
   A three-piece version of the above uses a tiny pivoting arm held under tension inside the hinge. Pulling on the handle from any direction lifts the pivoting arm's pins out of the zipper teeth so that the slider can move.
  
  Section::::Components.
  
  The components of a zipper are:
  
  BULLET::::1. Top Tape Extension (The fabric part of the zipper, that extends beyond the teeth, at the top of the chain.)
  
  BULLET::::2. Top Stop (Two devices affixed to the top end of a zipper, to prevent the slider from coming off the chain.)
   In many jackets and similar garments, the opening is closed completely when the slider is at the top end. 
  
  Some jackets have double-separating zippers with two sliders on the tape. When the sliders are on opposite ends of the tape then the jacket is closed. If the lower slider is raised then the bottom part of the jacket may be opened to allow more comfortable sitting or bicycling. When both sliders are lowered then the zipper may be totally separated. 
   In 1851, Elias Howe received a patent for an "Automatic, Continuous Clothing Closure". He did not try seriously to market it, missing recognition he might otherwise have received. Howe's device was more like an elaborate drawstring than a true slide fastener.
   These are metal devices that are affixed at the top and bottom ends of the zipper respectively, to prevent the slider from coming out of the zipper chain. The bottom stop also prevents the already interlocked zipper teeth from separating into half.
  
  BULLET::::- Insertion pin and retainer box
   BULLET::::- Coil zippers now form the bulk of sales of zippers worldwide. The slider runs on two coils on each side; the "teeth" are formed by the windings of the coils. Two basic types of coils are used: one with coils in spiral form, usually with a cord running inside the coils; the other with coils in ladder form, also called the Ruhrmann type. Coil zippers are made of polyester coil and are thus also termed polyester zippers. Nylon was formerly used and though only polyester is used now, the type is still also termed a nylon zipper.
   Bags, suitcases and other pieces of luggage also often feature two sliders on the tape: the part of the zipper between them is unfastened. When the two sliders are located next to each other, which can be at any point along the tape, the zipper is fully closed. 
  
  Zippers may:
  
  BULLET::::- increase or decrease the size of an opening to allow or restrict the passage of objects, as in the fly of trousers or in a pocket;
   BULLET::::- Bidirectional Use of Sliders: This is the biggest advantage of coil zippers. The slider can be incorporated on either direction and it will work properly. However, one of the directions is better than the other; they work quite well for both of them.
  
  BULLET::::- Self-Heal Property: While other zippers may become useless once they are impaired, nylon zippers have the tendency to self-repair. Even when some of the teeth misaligns and gets out of the regular positioning, moving the slider past that point will automatically reposition it to its original place.
   A Montreal firm, Velek, Ltd., acquired the exclusive right to market the product in North and South America, as well as in Japan, with American Velcro, Inc. of New Hampshire, and Velcro Sales of New York, marketing the "zipperless zipper" in the United States.
   The most recent innovation in the zipper's design was the introduction of models that could open on both ends, as on jackets. Today the zipper is by far the most widespread fastener, and is used on clothing, luggage, leather goods, and various other objects.
  
  Section::::Types.
   This refers to the device that is pulled up or down to open or close the zipper. It is what clasps the zipper teeth or separates them depending on the direction of movement. Attached to it is the pull tab, the extension that is held when moving the zipper up or down. Some sliders come with a zip-lock feature in that, when the pull tab is lowered, it locks the slider in place and when raised, it unlocks it.
  
  BULLET::::- Tape
   These are found exclusively on separating zippers. The pin box or retainer box aligns the insertion pin and the rest of the separating zipper to begin clasping of the zipper teeth. The insertion pin is the metallic end of the zipper that it placed into the retainer box to allow the joining of the two halves of the zipper.
  
  Section::::Manufacturers.
   Coil zippers are zippers whose teeth/elements are made from that was traditionally nylon. The coil is sewn to a zipper tape to make the final product. The final zipper product is completed when the nylon coiled teeth are sewn onto the zipper tape. Nylon coil zippers have a continuous coil chain made from nylon. When this coil is positioned on the back of the zipper and not in the front, this kind of zipper is called invisible zipper.
   The Hefty OneZip sliding tab sealable bag, formally known as the "rolling-action zipper profile and slipper", was created by Eric A. St.Phillips and F. John Harrington Jr. at Mobil in the early 1990s at a cost of $25 to $50 million, after having developed and evaluated various different zipper designs, and finally deciding to roll out the "boxcar-shaped version", which slides over dual plastic tracks.
  
  Section::::History.:Environmental concerns.
  
raw_presuppositions: Zippers should work when grabbed anywhere.
raw_corrections: Zippers have a mechanism that stops them from moving by themselves.