A cap for a container is formed so that the cap has a top plate and a circular sidewall. Two opposite sides of the circular sidewall circularly connect to each other, one periphery of the circular sidewall connecting to one surface of the top plate forming a closed end, and another periphery of the circular sidewall at an opposite side of the closed end forms an opened end. Incisions are in the circular sidewall. The incisions form a ring member located at the opened end of the cap separated from a main body of the cap by a first incision and a second incision between the opened end of the main body and the ring member.