Commonly used in laboratories because of their impermeability and general absence of toxic elements, cork is a useful material to stop test tubes and flasks. Cork, also known as a stopper or bung, is partially inserted inside of a container to form a seal. This differs from a lid, which does not displace the inner volume during a seal. Cork stoppers, which come in numerous sizes, are generally safe for use in packaging or for the processing of food and are best suited for solid specimens and materials. Corks should be rolled on a clean surface to soften their lower half prior to use, allowing the cork to expand and form a tight seal when pressed into a flask or vial.