@jq Mercury is used in old thermometers because it expands consistently with temperature forcing a fluid to move up a tube to different temperature levels. If it's exposed to air, it evaporates at room temperature, when heated, it evaporates faster. The inhalation of mercury vapour can produce harmful effects on the nervous, digestive and immune systems, lungs and kidneys, and may be fatal. It might be safer to eat the sushi than to stand over a tuna or salmon when cooking it.