Mercury is both a liquid and a metal at ambient temperature (due to its extremely low melting point of -39C). We have a tendency to believe that metals are all solid, but this is not the case. Even though mercury is a heavy element it can be fluid due to its bond angle which allows for more stable configurations than those available in lighter elements.
It was once believed that only noble gases were gaseous at standard temperature and pressure (STP), but this is not true. Several metals such as potassium, sodium, magnesium, calcium, and aluminum have very low vapor pressures at STP. However, these metals easily dissolve in liquids and their atoms displace molecules of the solvent.
The first element discovered that has a measurable vapor pressure at STP is hydrogen. All other elements have vapor pressures so small that they cannot be measured under these conditions.
Mercury is a silvery, thick d-block element. It is the only metal that is liquid under conventional temperature and pressure conditions. Mercury has one of the smallest liquid state ranges of any metal, with a freezing point of -38.83 deg C and a boiling point of 356.73 deg C. Because it is so reactive, most mercury is kept in a safe container away from oxygen and other elements that could chemically change it.
Older thermometers contained some amount of mercury because it was inexpensive and gave reliable readings down to 0 degrees Celsius (32 degrees Fahrenheit). Modern thermometers usually contain silicon instead, but they still work if you bring them into contact with mercury.
There are several ways that mercury can be used as a heat transfer medium. The simplest method is to pour the liquid over hot stones or another heat source. This is called "calcining". The stone or other heat source heats the mercury which then transfers its heat to whatever you are heating.
You can also use mercury to cool things off. A piece of copper or aluminum will absorb some of the mercury's heat when it is in contact with it. When you remove the metal object from the mercury it will no longer be hot because the metal's heat capacity is too great for the mercury to lose much if any of that heat.
Finally, you can use mercury to do work.
Mercury has a melting point of -38.9 degrees Celsius, a boiling point of 356.7 degrees Celsius, and is the only metal that remains liquid at ambient temperature. Droplets of liquid mercury are gleaming and silver-white, with a strong surface tension that makes them look spherical when placed on flat surfaces.
When exposed to air, mercury will gradually evaporate, leaving small particles of elemental mercury that are highly toxic. The particle size decreases as more mercury is evaporated, so even very tiny amounts of mercury can be harmful if they are inhaled or ingested.
The human body cannot absorb elemental mercury; it must be converted into another form before it can be eliminated. This process happens primarily in the liver where chemicals called sulfurs combine with mercury to create compounds that are then removed from the body in urine or feces.
People may come into contact with mercury for different reasons. It may be found in amalgam (75% mercury by weight) used by dentists to fill teeth because it has many advantages over other materials. Amalgam is soft and malleable which allows your dentist to shape it into any number of useful shapes. It also changes color when heated indicating that it does not melt all the way through like other metals.
Another source is fish containing methylmercury, which is toxic. In fact, eating contaminated fish is one of the most common ways people become aware of the effects of mercury on humans.