As a kid, I used to love to play with mercury. It is the only metal that is liquid at room temperature, and as a junior chemist I enjoyed chasing little balls of glistening silver as they split into bits and then gathering them into a larger mass. This was more fun than any 10-year-old could imagine — at least any 10-year-old nerd chemist.
Mercury was used in those days in thermometers, blood pressure machines, vaccines and for a variety of industrial uses. It currently gets into the ocean from power plants and volcanic activity.