At liquid-nitrogen temperature, about –320°F, mercury acts like any other metal: You can hammer it, file it, saw it. (It won't shatter like other liquid-nitrogen-frozen items because there's not enough moisture inside.) Watching it solidify is exactly like watching tin harden from a molten state. As the atoms go from liquid to solid crystal form, you see the surface pucker. And because mercury, like most metals, shrinks when it solidifies, you see the surface sink in areas, forming a patchwork characteristic of cast metal.