The fruit of the walnut tree, which grows in temperate zones throughout the world. The two most popular varieties of walnut are the ENGLISH WALNUT (also called Persian walnut) and the BLACK WALNUT. A close relative is the BUTTERNUT, also referred to as white walnut. ENGLISH WALNUT are the most widely available and come in many varieties-some with moderately thick shells, others with shells so thin a tiny bird can crack them open. The nuts of all the species are edible, but the walnuts commonly available in shops are from the common walnut, the only species which has a large nut and thin shell. A horticultural form selected for thin nut shells and hardiness in temperate zones is sometimes known as the 'Carpathian' walnut. The nuts are rich in oil, and are widely eaten both fresh and in cookery. Walnut oil is expensive and consequently is used sparingly; most often in salad dressing. Walnuts are also an excellent source of omega-3 fatty acids, and have been shown as helpful in lowering cholesterol. They need to be kept dry and refrigerated to store well; in warm conditions they become rancid in a few weeks, particularly after shelling. Oil paint often employs walnut oil as an effective binding medium, known for its clear, glossy consistency and non-toxicity.