frangible areas of different configurations are formed between the cap and skirt portions of a unitary molded plastic closure , with an internal continuous groove formed on the inside surface of the skirt portion having a diameter slightly larger than the outer diameter of an upper cap portion , the groove having an inner diameter slightly larger than the outer diameter of the cap , the groove lockingly engaging in a tight - fitting manner with the outer peripheral wall of the annular collar on the neck of the container when the closure is assembled with the container , and the groove together with ribs formed proximate to the groove provide a locking engagement between the container neck and the lower skirt portion , facilitating forming an annular collar on the container neck with an outer diameter larger than the outer diameter of the upper cap portion so as to prevent the closure from being pried off by children during use of the contents of the container .