Abstract: 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. Incision lines are in the circular sidewall. The incision lines form a ring member located at the opened end of the cap separated from a main body of the cap by a first incision line in between the opened end of the main body and the ring member. The ring member is connected to the main body by a plurality of connecting portions.