Threadlockers and retaining compounds are adhesives that hold mechanical parts together to resist vibration. Threadlockers work with threaded fasteners while retaining compounds work with unthreaded parts. Threadlockers for metals are the most commonly used. They are anaerobic, which means they remain fluid until they fill the empty space between threads and then harden in the absence of air and in the presence of metal ions. Threadlockers for plastics are non-anaerobic and harden quickly when exposed to air. Reusable threadlockers for metals and plastics do not harden but remain soft and flexible for metal and plastic fasteners that require removal, adjustment, and reuse. Retaining compounds for metals are thin, anaerobic adhesives that seep into tightly machined cavities of unthreaded metal parts to hold them in place and dampen vibration.