Abstract: A cap to close off an externally threaded aperture of a container is provided. The cap includes a base wall and a peripheral skirt carrying an internal thread for mating with the aperture's external thread. A hollow spigot portion extends from the base wall within the cap, and is co-axial with the skirt and attached to the base wall by a frangible connection. Grooves or thread are formed on the outside wall of the spigot portion to engage the wall of the bore in the aperture. A method of sealing a container including engaging a cap is also provided.