A bottle cap for universal retrofitting to one of a number of pre-existing beverage containers containing a beverage and having a removed cap and threaded aperture, having at least one container portion for containing at least one dispensable material, a threaded portion for receiving the threaded aperture and creating, when threaded, a watertight assembly; and bag containing the dispensable material and located in the container portion. Upon screwable attachment of the threaded aperture into the cap, the threaded aperture parses at least a portion of the bag to permit feeding of the material into the beverage. A plurality of such caps are also shown packaged for commercial sale. The material contained in the cap is selected from the group consisting of vitamins, medicaments, teas, flavors, concentrates and water-soluble food material. Also shown is a universal cap further having a moveable aperture assembly creating an open, fluid-flowing state and a closed, fluid-stopped state, having a top portion containing a drinking aperture and an elongated, moveable fluid-flowing portion.