The screw's the thing: it should be a reasonably thin coil that comes to a definite point that can work its way firmly into the cork. Any number of corkscrews will open a bottle adequately, from the almost foolproof, Teflon-coated screwpull model, to the levered butterfly design, whose arms are lowered as the cork rises, to the trusty waiter's corkscrew with its small knife and single-levered action, to a screw-less variety with two thin prongs that remove a cork intact with a pulling-twisting motion.