Gallium

Gallium is an elemental metal with the exceptionally low melting point of just 30°C. Because skin temperature is higher than this, simply holding a chunk of gallium in one’s hand provides sufficient energy to break the weak bonds that hold the atoms together, allowing melting to occur and the metal to become liquid. By mixing gallium with the low melting point metals indium and tin (termed Galinstan alloys) the resulting material has a melting point of just -19°C, making it a liquid at room temperature. However, Galinstan does not boil and evaporate until approximately 1300°C, making it a suitable, and safer, replacement for mercury in thermometers, since it provides a wide temperature range over which it is liquid, without presenting the toxicity of mercury. 

Gallium can also be used for the creation of highly reflective homemade mirrors; on smearing a small amount onto a glass slide, gallium spreads across (or ‘wets’) the surface, creating a perfectly mirrored finish. Because of the low melting point, these effective devices can be created very simply, leading to gallium being exploited for mirrors in space telescopes.