A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger and, if sufficiently bright, may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many cultures.

Community of Metros (also called CoMET) is a system of international railway benchmarking. CoMET consists of large metro systems from around the world. The 16 members of CoMET are the Beijing Subway, Berlin U-Bahn, Guangzhou Metro, Hong Kong Mass Transit Railway, London Underground, Mexico City Metro, Madrid Metro, Moscow Metro, New York City Subway, Paris Métro, Santiago Metro, Shanghai Metro, São Paulo Metro, Taipei Metro, Singapore Mass Rapid Transit and Delhi Metro. Each metro has a volume of at least 500 million passengers annually.