Rubber bottle stoppers, sometimes called rubber corks or tapered rubber plugs, are used to seal bottles, test tubes, flasks, and other laboratory containers. Stoppers are generally tapered in shape, though some are shaped with a wide, rounded top. They come in a variety of standard sizes and fit inside the neck opening of a container to provide a tight seal that helps prevent spills and evaporation. Some rubber stoppers with a hole allow glass tubing to be inserted into the container through the stopper. Drilled stoppers may have one or two holes for tubing. Stoppers are usually made from natural rubber for durability, flexibility, and water resistance, but some are made of neoprene for added resistance to some chemicals such reagents or solvents that would damage rubber.