Abstract: The present invention provides a cap for covering the open top of a drinking vessel. The cap includes a substantially rigid cover disk assembly dimensioned to span across and substantially cover the open top of the drinking vessel, and a flexible tubular membrane that extends from a bottom side of the cover disk assembly. The membrane is adapted to be rolled down a side wall of the drinking vessel to thereby removably secure the cap thereto. Identifying indicia can be printed on the top side of the cover disk assembly. The beverage within the drinking vessel can be consumed using a drinking straw. When properly deployed, the cap inhibits the introduction of unwanted matter into the drinking vessel.