What is a common method used for lubricating mechanical systems?

Applying oil or grease is a widely adopted method for lubricating mechanical systems due to its effectiveness in reducing friction between moving parts. These substances create a thin film that adheres to surfaces, providing a barrier that minimizes wear and tear, prolonging the life of components. Oil is particularly beneficial because it can flow easily to reach all necessary areas, while grease, being thicker, is ideal for situations where lubrication needs to stay in place longer or where leakage must be minimized.

In contrast, using water as a lubricant is not practical for most mechanical systems, as water can promote rust and corrosion, and it lacks the necessary lubricating properties found in oils and greases. Utilizing sand as an abrasive would actually cause damage to mechanical components rather than lubricate them, leading to increased friction and wear. Emitting air pressure does not serve the purpose of lubrication; instead, it might be used for cleaning or moving debris but does not provide the necessary reduction in friction that lubricants do.