Bottle caps provide an air-tight seal that ensures that a bottle’s contents are leak- and spill-proof, resistant to temperature and that the bottle can be moved. The most ubiquitous style is the threaded screw cap. Caps are available in different sizes and colors and can be specialized for the type of container they sit upon. Popular cap materials include phenolic plastic, polypropylene and PTFE. Bottle caps can be lined with additional material and can be found in solid design or open design, which calls for a septa or insert for complete closure.