This is what a lightning rod looks like after it's done its duty. Why does a lightning bolt crush it like a fist crushes a beer can? Because of what's known as a "pinch."
The phenomenon is like a much more prosaic sight — a stream of water from a sink breaking into beads. Gravity pulls the water down into a line. Surface tension squeezes the water inwards, breaking the line into drops. Similarly, as electric current moves through a conductor, it creates a magnetic field that pinches the conductor inwards, crumpling the plasma into drops, or the pipe into an hourglass.
This pinch has been shown to work on plasma as well. Plasma is a substance created when, due to high voltage or high temperatures, electrons come away from their atoms and the two swim around together in a kind of soup. Plasma is a great conductor for electricity, and often gives off light, which is why people noticed this pinch phenomenon squeezing a "stream" of plasma into separate beads.
[Via Thunderbolts.Info]