It's like a little ridged "pin" on the flower that fits into the hole in the stem.
Push gentle down the pin in the hole so the flower is attached. Keep in mind that plastic jewelry is NOT as durable as metal jewelry. If you change the top often or take the jewelry in and out often, the labret can become too stretched and the top does not fit as well anymore. Loose rods without top can be bought in all sizes, search for article number: BIO053RO
Bioplast® is a form of medical "plastic" that is safe to use in piercings. It is an excellent material for final piercings and for those with sensitive skin due to its biocompatibility and flexible nature.
Bioplast® is a trademarked plastic of medical quality. It was created in 1998 in Austria and is still manufactured in Tyrol. It was previously sold under the name PMFK until 2003 when the name was changed to Bioplast®. Bioplast® is a biocompatible polymer, flexible and can be cut to any length and threaded with any metal ball.
Bioplast® adapts to body temperature and not to outdoor temperature, like metals. Therefore, people who get pierced with Bioplast® are less likely to get swelling and infections. The healing process is faster than with any other material and there are no registered allergic reactions. The material is considered suitable for new piercings of APP.