Abstract: The lid for a cup has a flange, an outer retainer, an inner retainer, a mounting groove and a protrusion. The cap covers on a cup and has a rim. The flange protrudes from the rim of the cap and has an outer edge and an inner edge. The outer retainer protrudes perpendicularly from the outer edge of the flange and has an inside surface. The inner retainer protrudes perpendicularly from the flange near the inner edge of the flange and is slanted toward the outer retainer. The mounting groove is formed in the flange and defined between the outer retainer and the inner retainer. The protrusion is formed on and protruding from the inside surface of the outer retainer and extends into the mounting groove.