O-rings are solid-rubber seals that are shaped like a doughnut. O-rings are used for sealing liquids or gases against leakage and loss. It is a loop of elastomer with a round 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. The joint may be static, or in some design that have relative motion between the parts and the o-ring. Joints with motion usually require lubrication of the o-ring to reduce wear. This is typically accomplished with the fluid being sealed. 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.