A keychain (/ˈkitʃeɪn/ ) (also keyring) is a small ring or chain of metal to which several keys, or fobs can be attached. The terms keyring and keychain are often used interchangeably to mean both the individual ring, or a combined unit of a ring and fob.
The length of a keychain or fob may also allow an item to be used more easily than if connected directly to a keyring. Some keychains allow one or both ends to rotate, keeping the keychain from becoming twisted, while the item is being used.
In chemistry, mechanically interlocked molecular architectures (MIMAs) are molecules that are connected as a consequence of their topology. This connection of molecules is analogous to keys on a keychain loop. The keys are not directly connected to the keychain loop but they cannot be separated without breaking the loop. On the molecular level, the interlocked molecules cannot be separated without the breaking of the covalent bonds that comprise the conjoined molecules; this is referred to as a mechanical bond. Examples of mechanically interlocked molecular architectures include catenanes, rotaxanes, molecular knots, and molecular Borromean rings. Work in this area was recognized with the 2016 Nobel Prize in Chemistry to Bernard L. Feringa, Jean-Pierre Sauvage, and J. Fraser Stoddart.
The synthesis of such entangled architectures has been made efficient by combining supramolecular chemistry with traditional covalent synthesis, however mechanically interlocked molecular architectures have properties that differ from both "supramolecular assemblies" and "covalently bonded molecules". The terminology "mechanical bond" has been coined to describe the connection between the components of mechanically interlocked molecular architectures. Although research into mechanically interlocked molecular architectures is primarily focused on artificial compounds, many examples have been found in biological systems including: cystine knots, cyclotides or lasso-peptides such as microcin J25 which are proteins, and a variety of peptides.
A good luck charm is an amulet or other item that is believed to bring good luck. Almost any object can be used as a charm. Coins, horseshoes and buttons are examples, as are small objects given as gifts, due to the favorable associations they make. Many souvenir shops have a range of tiny items that may be used as good luck charms. Good luck charms are often worn on the body, but not necessarily.