. Lips are wonderful things: useful, decorative, sensitive, erogenous. Many of the best things that life has to offer are experienced through the lips: fine wine, fancy cakes, a consensual kiss, crack cocaine. They are marvellous, fabulous devices, like sculpted scar tissue, like little chipolatas, like a soft seagull of sensual promise. Parts of the lips have evocative names like the vermilion border, and the cupid's bow. They are also full of nerves and blood vessels and muscles, and the skin there is thinner, making these perfect, pink protuberances extremely vulnerable to twisting, pulling, scratching, pinching and tearing. They don't like being bitten either.
In body part terms, they are weak, effete, as if their epicurean life has made them weak and decadent, like swooning dandies. Punish them, test them - they will not stand up to any great scrutiny. Oh, and sometimes lips have hair attached to them, in the form of a moustache or goatee / standard beard. This hair can be pulled, causing eye watering discomfort. A most effective technique.