An improved cap is provided for sealing an opening formed in an associated bottle. The cap is preferably internally threaded for receipt over a threaded portion of the bottle. A flap extends from a planar surface of the cap and is directed for receipt in the associated bottle opening. A shoulder also extends outwardly from the planar surface and is spaced inwardly from the flap to engage the seal flap as the cap is threaded onto the bottle. The cap is preferably formed of a rigid plastic such as high density polyethylene (HDPE).