In recent years, air travel regulations have only gotten stricter. But as the list of banned substances on airplanes continues to grow, one item has been on the top since the beginning, and will likely never be taken off this list. That item is mercury.
Mercury is, curiously, the only metal that's a liquid at room temperature. This property, along with its high coefficient of thermal expansion, makes mercury ingeniously useful in thermometers. That is, the mercury is highly sensitive to changes in heat and expands a lot in response, thereby allowing you to accurately gauge the temperature.
Mercury is also highly reactive with other metals, including pure aluminum - the stuff that makes up the bulk of planes. When mercury finds pure aluminum, it causes severe changes in the chemical and physical properties of the metal. In essence, the mercury will "eat" through the aluminum rapidly. And the reaction will continue until either the mercury evaporates or the aluminum is gone.
So imagine a scenario where a mercury thermometer is brought on the plane, and the glass breaks. Even the tiny amount of mercury that's released can pose deadly risks to the plane. And that's why we should all keep our thermometers at home when traveling.