A cap for a container with an opening, the cap comprising: (a) a base having a round platform with an axis, a platform top surface with a perimeter, a platform bottom surface, a container attachment on the platform bottom surface, and a through aperture extending from the platform top surface to the platform bottom surface; (b) a cylindrical tank having a region wall dividing the tank into a compartment region with a top having a closure, and a platform region, the compartment region including a plurality of compartments and at least one vacant location arranged in a circle around the axis, each compartment having a compartment through opening in the region wall covered by a frangible seal, at least one of the compartments containing an additive, the at least one vacant location not containing an additive, the tank mounted to the base by an attachment mechanism such that the tank rotates about and reciprocates on the axis relative to the base, the tank being rotatable on the axis such that the compartment openings and the at least one vacant location can align with the aperture; and (c) a spike extending from the aperture toward the region wall; characterized in that the cap further comprises: (d) a biasing mechanism biasing the tank toward the base; (e) a series of truncated teeth extending upwardly from around the platform perimeter above the top surface of the platform into the platform region, a profile of each tooth having a generally flat valley, a ramp extending upwardly at an angle from the valley to a plateau, to a vertical wall, and down to the valley of the adjacent tooth; (f) at least one cog extending downwardly into the platform region to ride on the teeth; and (g) the tank rotating and reciprocating on the axis between a selecting position wherein the cogs ride up the ramps to the plateaus, thereby pulling the tank away from the base against the biasing mechanism, and an operative position wherein the biasing mechanism causes the cogs to drop down the vertical walls to the valleys, thereby causing the spike to pierce the frangible seal associated with a compartment aligned with the aperture, thereby allowing the additive to flow downwardly into the aperture and into the container attachment. (a) a base having a round platform with an axis, a platform top surface with a perimeter, a platform bottom surface, a container attachment on the platform bottom surface, and a through aperture extending from the platform top surface to the platform bottom surface; (b) a cylindrical tank having a region wall dividing the tank into a compartment region with a top having a closure, and a platform region, the compartment region including a plurality of compartments and at least one vacant location arranged in a circle around the axis, each compartment having a compartment through opening in the region wall covered by a frangible seal, at least one of the compartments containing an additive, the at least one vacant location not containing an additive, the tank mounted to the base by an attachment mechanism such that the tank rotates about and reciprocates on the axis relative to the base, the tank being rotatable on the axis such that the compartment openings and the at least one vacant location can align with the aperture; and (c) a spike extending from the aperture toward the region wall; characterized in that the cap further comprises: (d) a biasing mechanism biasing the tank toward the base; (e) a series of truncated teeth extending upwardly from around the platform perimeter above the top surface of the platform into the platform region, a profile of each tooth having a generally flat valley, a ramp extending upwardly at an angle from the valley to a plateau, to a vertical wall, and down to the valley of the adjacent tooth; (f) at least one cog extending downwardly into the platform region to ride on the teeth; and (g) the tank rotating and reciprocating on the axis between a selecting position wherein the cogs ride up the ramps to the plateaus, thereby pulling the tank away from the base against the biasing mechanism, and an operative position wherein the biasing mechanism causes the cogs to drop down the vertical walls to the valleys, thereby causing the spike to pierce the frangible seal associated with a compartment aligned with the aperture, thereby allowing the additive to flow downwardly into the aperture and into the container attachment.