The Black market

The Black market or underground market is the part of economic activity involving illegal dealings, typically the buying and selling of merchandise or services illegally. The goods themselves may be illegal to sell ; the merchandise may be stolen; or the merchandise may be otherwise legal goods sold illicitly to avoid tax payments or licensing requirements, such as cigarettes or unregistered firearms. It is so called because "black economy" or "black market" affairs are conducted outside the law, and so are necessarily conducted "in the dark", out of the sight of the law.
Black markets develop when the state places restrictions on the production or provision of goods and services. These markets prosper, then, when state restrictions are heavy, such as during a period of prohibition, price controls and/or rationing. However, black markets are currently present in any known economy.

The International System of Units is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science.
The older metric system included several groupings of units. The SI was developed in 1960 from the metre-kilogram-second system, rather than the centimetre-gram-second system which, in turn, had many variants.
The SI introduced several newly named units. The SI is not static; it is a living set of standards where units are created and definitions are modified with international agreement as measurement technology progresses.
With few exceptions, the system is used in every country in the world, and many countries do not maintain official definitions of other units. In the United States, industrial use of SI is increasing, but popular use is still limited. In the United Kingdom, conversion to metric units is official policy but not yet complete. Those countries that still recognize non-SI units have redefined their traditional non-SI units in terms of SI units.

Smoke

Smoke is a suspension in air of small particles resulting from incomplete combustion of a fuel. It is commonly an unwanted by-product of fires and fireplaces, but may also be used for pest control , communication , defense or inhalation of tobacco or other drugs. Smoke is sometimes used as a flavouring agent and preservative for various foodstuffs. Smoke is also sometimes a component of internal combustion engine exhaust gas, particularly diesel exhaust.
Smoke inhalation is the primary cause of death in victims of indoor fires. The smoke kills by a combination of thermal damage, poisoning and pulmonary irritation caused by carbon monoxide, hydrogen cyanide and other combustion products.
Smoke particles are actually an aerosol of solid particles or liquid droplets that are close to the ideal range of sizes for Mie scattering of visible light. This effect has been likened to three-dimensional textured privacy glass—the smoke cloud does not obstruct an image, but thoroughly scrambles it.