Bottle caps are small, typically circular pieces of plastic or metal that seal the openings of bottles. They are designed to keep the contents fresh and prevent spills. Most bottle caps have a threaded design that allows them to screw onto the neck of the bottle, while others may use a snap-on mechanism. These caps come in various sizes and styles, depending on the type of bottle they are used for, such as those for soda, water, or beer. Some bottle caps also feature a liner to create an airtight seal, ensuring the beverage remains carbonated or protected from contamination.