Which metal is liquid at room temperature?

Mercury melts at about minus 39 degrees Celsius, so at anything like room temperature it is a liquid, and a startlingly heavy one - a litre of it weighs over thirteen kilograms, and a cannonball would float on top.

The reason is genuinely exotic. Mercury's nucleus is so heavily charged that its innermost electrons have to move at a substantial fraction of the speed of light, and relativity makes them behave as though heavier, contracting their orbits and pulling the outer electrons in tight. Those outer electrons are then held so firmly by their own atom that they barely participate in bonding with neighbouring atoms. The metal is liquid because relativity makes its atoms antisocial.

It is also poisonous, which took an unreasonably long time to matter. Hat-makers used mercury compounds to treat felt and suffered tremors, irritability and cognitive damage - the origin of the phrase mad as a hatter. One caution on the question: it depends on the room. Gallium melts at about 30 degrees and will liquefy in a warm hand, and caesium at about 28.