A suction-cup dildo has a concave base that sticks to smooth, nonporous surfaces: tile, glass, some acrylic, a clean bathtub wall. Hands come off the toy. Hips do the work. Showers become occupied territory.

The cup only holds if the surface is glossy and wet enough to seal, or clean and dry, depending on the silicone. Matte paint, unfinished wood, and textured tile will not play along. A cup that slowly walks down the wall is telling you the seal is marginal. Re-seat it before you trust a squat.

Some cups unscrew from Vac-U-Lock sockets so the same shaft can go in a harness. That modularity is useful. Check the thread. A shaft that spins off the cup mid-ride is a classic, unfunny moment.

Mounting

• Press from the center of the cup outward.
• Keep a grab rail in the shower that is not the towel bar.
• Wash the cup face; dust kills suction.
• Do not stick a heavy steel toy to a cheap suction adapter and hope.