[Scene: A windy night, driving home from tango class.  We stop at a light, and the traffic light poles are swaying violently back and forth.]

Suzi: Wow...I don't think they are supposed to move that much.

Me: Actually, they are designed to give.  If they didn't they'd be much more likely to fracture or collapse. 

Suzi: Really?

Me: Yeah, tall buildings are the same way.  In an earthquake or a hurricane, the buildings are designed to sway back and forth without falling down.

Suzi: [surprised] Huh.

Me: Yeah. My high school physics teacher told me that the World Trade Center used to sway back and forth several feet because of the sun.  In the morning, the sun would heat up the east side of the building and the metal would expand on that side, and the building would lean like this [hand motions indicating leaning to one side], and then in the afternoon, the other side would heat up, and it would lean the other way.

[A few seconds go by]

Suzi: Yeah...I guess they couldn't design for those planes, though.