What are receding gums? Teeth often become sensitive when the gum recedes because of age, disease or over-vigorous brushing. The newly exposed dentin is more porous than the rest of the tooth surface and new pathways open to the nerves inside the pulp, resulting in sensitivity to hot and cold things. The sensitivity may go away by itself, but some over-the-counter products can help. One theory is that the pain is caused by movement of fluid filling the tiny tubes in the dentin that open into the pulp chamber. The fluid might stimulate the nerves near the dentin-pulp border. According to this theory, the pain might be lessened by blocking the tubules thereby blocking the nerve impulses that cause pain. How does a match light up? The friction created by dragging a match across a rough surface creates enough heat to burn phosphorus sesquisulfide. This is what is at the tip of a non-safety match. Safety matches (which include books of matches) have a head made of several ingredients, the main being chlorate of potash. The safety match is so called because it works best when it is used on a special striking surface - made of phosphorus and sand. Matches really began to be popular in the early to mid-1800s. About 1830, an Englishman named John Walker invented a 'safe' phosphorus-free match that he called a 'friction light'. Do you know what the fruit from the ginkgo tree is like? In China, where the ginkgo has been cultivated for nearly 1,000 years, the nuts are prized for their medicinal use. In the West, the tree is appreciated for its ornamental, fan-shaped leaves. Raw ginkgo nuts are considered toxic, but they are used in traditional Chinese herbal medicine. When cooked, the nuts taste like sweet chestnuts. The ripe, raw nuts have a foul odour and can also cause a severe skin rash. The odour is not a problem when the nuts are harvested for consumption, because they are washed of their smelly, fleshy outer coat. Today only the male tree, which does not bear nuts, is used for ornamentation. Centuries ago in China, ginkgo leaves and their extract were used to treat skin and head sores and to relieve asthma and cough. In the 1980s in Europe, the leaves were found to contain compounds that increase blood flow in people with circulation problems. Are pollutants linked to cancer? Some common environmental pollutants that are linked to cancer may start their dirty work by first weakening the body's immune defence system, a team of researchers has found. Laboratory studies showed that polycyclic aromatic hydrocarbons - which are found in car exhaust, cigarette smoke, charcoal-grilled and smoked meat and fish - can kill white blood cells called immature B lymphocytes.