Cap plugs and threaded caps are used to close laboratory containers, providing a tight seal that helps prevent spills and evaporation. Plastic caps and plastic plugs are made to fit standard neck sizes of bottles and tubes. Plastic end caps are designed for sealing tubing and pipe ends. Cap plugs snap into place to close, and screw caps are threaded and twist into place. Plugs and caps are commonly made of plastic, including polypropylene, polyethylene, high-density polyethylene, or polysulfone, among others, but they may also be made of stainless steel or aluminum. Some caps have a liner made of foam, polytetrafluoroethylene, rubber, silicone, foil, or polyethylene. A cap with a septum is used for injectable medications or vaccines, and a cap with a vent allows for gas exchange while protecting the contents from contamination.