Glass Production Puns

So i know the joke is old, but the application was classic...

I was with my dad buying some glass cutting supplies and the guy in the store was really dry and seemed to take his job pretty seriously. We wrapped up our purchase and the shop worker asked if we wanted a quick tour of the production facility. We said 'yes' and walked into the back. Shop worker guy showed us some bullet proof glass, and a new self obscuring glass... then he mentions that most of what they do is provide mirrors for elevators and he turns to walk us over to the last section of the facility where they do wood grain backed mirrors. My dad stops walking and says "elevators hey"? The shop worker turns and looks at him... "I hear that industry has it's ups and downs"... Shop worker guy makes eye contact with me and turns and continues his tour. While no noise escaped his lips i could feel an internal groan that shook the walls.