a plastic bottle cap for use in sealing bottles , the cap having a first annular sealing flange engaging the internal wall surface of the bottle and a second annular sealing flange engaging the top surface of the bottle neck . the second flange is constructed with a flexible tip which flexes relative to the remaining portion of the second flange as it is engaged by the top of the bottle neck to produce a flapper type seal . the cap further includes a tamper - proof ring at its bottom end which is automatically torn away as the cap is unthreaded from the bottle .