I must have delicate lips or something. When a plastic stem gets a little scuffed I feel it—sandpapery—and I don’t like it. I must also have sharp teeth. I don’t often bite the stems, and I don’t chew them. I let the stem hang between my teeth, and that, and the weight/length of the pipe, is enough to rough them up pretty quickly. I’ve never bitten through an mm stem or squeezed shut the airway. But I do have a problem with these stems developing cracks down the length of the stem. Guess I’ll just double up on the replacement stems I’m about to order.