Mercury is an unusual substance: it's a metal that's liquid at room temperature. This makes it quite useful in things like thermometers...and it makes for fun lab tricks if you've got the right equipment.
In the video by the Royal Institution below, Olympia Brown makes a hammer out of mercury. This is possible because mercury freezes solid at -38.83 degrees Celsius, and liquid nitrogen is far colder than that. From there, she proceeds to hammer similarly frozen rubber "nails" (just pointy slivers of rubber) using the mercury hammer. I can guarantee that this is the first time I've seen a mercury hammer.
Along the way, of course, Brown explains how temperature works in relation to different substances. It's all about the third law of thermodynamics: The entropy of a perfect crystal at absolute zero is exactly equal to zero. Tune in, my fellow nerds!