Explain why the roof of an underground mine...

Explain why the roof of an underground mine can fall without warning. The audience is a new underground worker who has been told to report loose roof but does not know what he is looking for. Open on the concrete moment: a shift passes under the same stretch of roof twenty times and nothing happens, and on the twenty-first pass it comes down. Then show what was changing above them that nobody could see, cracks spreading through the rock while the surface looked the same. Turn on what makes the difference, the support installed before anyone works under a new section, and the sag and sound that warn before a fall. Land on the rule that follows, never work under unsupported roof, no matter how solid it looks.