a dual - compartment container for use with two substances which must be separated prior to use and which may be combined by the single step of removing the cap from the container . a top container containing one substance is sealed along its bottom end by a membrane and is rotatably seated within a bottom container containing another substance . a container cap seals the top container prior to use and is threaded onto the bottom container . the cap is provided with a ratchet means which causes common rotation of the cap and the top container while the cap is being unthreaded from the bottom container . an annular knife member is interposed between the two containers and keyed to the top container for common rotation therewith as the cap is removed . cam means are provided to translate rotary motion of the knife member relative to the bottom container into longitudinal motion of the knife member relative to the top container in order to pierce the sealing membrane .