Lips are famously sensual but, together with the gums and tongue, they are also surprisingly sensitive, second only to the fingertips in nerve density. Researchers at Carnegie Mellon University have exploited this latter characteristic to devise a practical way for people to receive tactile feedback in virtual worlds.   Their system uses airborne ultrasound waves to create sensations on the lips, teeth and tongue, and is small and light enough to attach to the bottom of virtual reality (VR) goggles.   Imagine a VR world that has a drinking fountain in it. "E...