It is made of zinc alloy and is safe for penis use, no sharp edge in every plugs, works good with some lubricant,temperature stimulation if you want the different feelings, cooled or iced with hot and cold waterSmooth surface, hollow in the middle design for cum, no need to take it off,convenient for inserting and removing. this plugs set fits both beginners and elite users for their different needsThese penis plugs were designed to focus on urethral expansion,for which will not hurt the super sensitive muscles down urethra. The safety knob in the end effectively prevents the plug from falling into the urethral canal. you can hold the knob to insert and pull out easilySeven penis plugs training kit available,you can enjoy the fun with the suitable size, gradually increasing the size requirement, from beginner to an expertUnder Water Play: The penis plug are waterproof and can be used underwater, bringing you a more marvelous sexual experience and adding more fun to your sex life with your lover