What type of lubricant is often used when assembling threaded pipe joints?

When assembling threaded pipe joints, a lubricant is essential to ensure a proper seal and ease of assembly. The correct answer is paste, specifically a pipe joint compound or thread sealant paste. This type of lubricant is designed to fill the small gaps and imperfections in the threads, preventing leaks in the final assembly.

Paste lubricants provide a few key advantages that make them ideal for threaded joints. They remain in place during tightening, offer excellent adhesion to both metal and PVC threads, and can withstand the pressures and stresses typically encountered in plumbing applications. Additionally, they often contain substances that help inhibit corrosion, which is crucial in environments where moisture may be present.

Considering the other options, grease and oil may not adhere well to the threads, potentially leading to a misalignment of the joints and leaks. Water, while sometimes used for cooling or cleaning, does not provide any sealing benefits and could actually assist in washing away other types of lubricants, making it an unsuitable choice. Thus, paste is the preferred and most effective lubricant for ensuring a tight and leak-free seal in threaded pipe joints.