A cap for a container, wherein the cap is adapted to be positioned across the mouth of a container, the cap comprising: an upper portion that lies across the mouth of a container; a sealed compartment for storing contents to be added to the container interior, the compartment being located below the upper portion, and defined by a lower membrane and a wall extending between the upper portion and the lower membrane, wherein the lower membrane is rupturable; and a piercer in the sealed compartment that is actuable through the upper portion to pierce the lower membrane, wherein the piercer is configured to form a passage therethrough to open the compartment to the container interior.