A hollow plug is open through the center. Once seated, it leaves a tunnel. People use that tunnel to add another toy, to look, or simply to feel stretch without a solid core taking up the same space.

Walls need enough thickness not to collapse, and the outer flare still has to be wider than the tunnel. Cheap thin tunnels fold and pinch. Medical-style tunnel plugs in steel hold a round bore. Silicone versions feel kinder and are harder to keep perfectly round under squeeze.

Anything you pass through the tunnel must be at least as clean as the plug. Edges of the inner lip should be rounded. A sharp inner rim will announce itself the moment something moves.

Cautions

• Do not force a second toy that matches the tunnel diameter with no slack.
• Remove the plug if the opening starts to feel numb.
• Clean the inner bore with a brush.
• Skip hollow plugs for long, unattended wear until you know the fit.