O-rings are simple. Super simple. According to Wikipedia:

An O-ring is a loop of elastomer with a round (o-shaped) cross-section used as a mechanical seal or gasket. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.

Now, many of use have used o-rings before. They are generally made of rubber or a rubber-like material. I have used them on plumbing repairs, lawnmower and car engine work, and even on stuff like fixing a thermos container.

O-rings are one of the most common seals used in machine design because they are inexpensive and easy to make, reliable, and have simple mounting requirements. It is one of the simplest, yet most engineered, precise, and useful designs ever developed.

They are so simple, but did you know there were so many types? Look at this:
Synthetic Rubbers:

• Acrylonitrile butadiene copolymers (NBR)
• Butadiene rubber (BR)
• Butyl rubber (IIR)
• Chlorosulfonated polyethylene (CSM)
• Epichiorohydrin (ECH, ECO)
• Ethylene propylene diene monomer (EPDM)
• Ethylene propylene monomer (EPM)
• Fluoroelastomers (FKM)
• Perfluoroelastomer (FFKM)
• Polyacrylate (ACM)
• Polychloroprene (CR)
• Polyisoprene (IR)
• Polysulfide rubber (PSR)
• Semi-Conductive Fluorocarbon with nano carbon tubes.
• Silicone rubber (SiR)
• Styrene butadiene rubber (SBR)

Thermoplastics:

• Thermoplastic Elastomer (TPE) styrenics
• Thermoplastic polyolefin (TPO) LDPE, HDPE, LLDPE, ULDPE
• Thermoplastic Polyurethane (TPU) polyether, polyester
• Thermoplastic etheresterelastomers (TEEEs) copolyesters
• Thermoplastic polyamide (PEBA) Polyamides
• Melt Processible Rubber (MPR)
• Thermoplastic Vulcanizate (TPV)

I have no idea what many of those are, but I am glad that a patent claim was applied for in 1937 by a then 72 year old Danish-born man, Niels Christensen. He never got the patent on the intellectual property rights, which is sad to say. A shame, I say. Without the o-ring, a good amount of the things we use and take for granted every day may never have been possible.

Thank you Niels, for inventing the o-ring. In many (mechanical) ways, it was as important as the microprocessor was to the Digital Age.

Related Links:
University of Houston Article on Niels Christensen
FAS Space Policy Project Challenger Accident website