Metals have been indispensable to humans since copper was first used some 9,000 years ago. They are prized for that blend of strength, hardness and heat resistance which gives them durability, and for their ease of shaping into complex forms. Improved technology has progressively reduced the price and improved the quality of metals, making them almost universally available and leading to improved living standards for countless people. Metals are used by every sector of society, and for a widening range of purposes. Basic industry critically depends on them for machinery and plant. No aircraft, automobile, computer, or electrical appliance can function without them. Aluminium packaging reduces losses in food storage and transport, and aluminium sailing boats provide a less-expensive alternative to traditional wooden craft. Chromium is essential both to the stainless steel, which provides hygienic work tops and utensils in modern kitchens and is also at the heart of the chemicals industry, and of many of the alloys on which commercial aviation depends. Substitutes, when they exist, are often more expensive than metals. For many purposes metals are incapable of substitution and are therefore irreplaceable; substitutes, when they exist, are often more expensive than metals.