Inexperienced naive sister, the diameter of the narrowest position is only 4mm! If the inner wall is too tight, it will stick to the penis. When you insert it to the deepest point, you will find that the innermost part is the widest and full of meat, and it will rub against the glans until the pleasure comes out! WARNING: Keep the product away from children. DO NOT expose the product to humid places, direct UV or sunlight for an extended period. Product is a reuseable after wash. DO NOT use this product with more than one person. DO NOT place any object inside the cup for use. Safety Precautions: Always clean the product before and after use. We recommend using water-based lube with your toy.