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Caesium is the most chemically reactive of all metals, except for the almost non-existing francium. It spontaneously burns in air and explodes when dropped in water.

When in the news, usually the radioactive and very dangerous 137Cs is meant. This is produced in nuclear power plants and has a half-life of 30 years. The natural 133Cs isn’t radioactive. Caesium is used in most atomic clocks, where a certain frequency of it is measured.