Red mercury is a hoax substance of uncertain composition purportedly used in the creation of nuclear bombs, as well as a variety of unrelated weapons systems. In reality, no such substance exists. It is purported to be mercuric iodide, a poisonous, odorless, tasteless, water-insoluble scarlet-red powder that becomes yellow when heated above 126 °C, due to a thermochromatic change in crystalline structure. However, samples of "red mercury" obtained from arrested would-be terrorists invariably consisted of nothing more than various red dyes or powders of little value, which may have been sold as part of a campaign intended to flush out potential nuclear smugglers.