Possibly daft question about expansion joints
The exhibit at the Seattle Museum of Flight (Alpha Golf, I think) has the story of the flight engineer who placed his cap in what's called "the expansion joint."
But I can't figure out how the "expansion joint" got itself closed when the airplane was cold, and open when the airplane was hot. Seems backwards to me.
In my mind, the cool airplane has smaller parts, and therefore larger gaps in the joints.
What am I missing?
(I shudder to ask this question, since I have the tingle on the back of my neck that usually tells me I'm missing something simple.)