Caesium

element · alkali metal · reactive · liquid metal

Melts at body heat. Caesium is so soft that it melts at just 28.5 degrees Celsius, which is about your body temperature! It is one of only five metallic elements that can be liquid near room temperature.

Explodes with water. Caesium is incredibly reactive and will actually react with water even at temperatures far colder than a freezer (negative 116 degrees Celsius). This makes it one of the most reactive stable elements known.

Biggest atomic size. Caesium has the largest atomic radius of any element scientists have measured or calculated, making its atoms unusually large and spread out.

Only one stable version. While scientists have created different forms of caesium atoms, only one version called caesium-133 naturally stays stable forever without breaking apart.

Used in reactors. Caesium-137 is made inside nuclear power plants and is extracted from reactor waste, making it useful for certain scientific and medical applications.