"What time are you open till?" is a seemingly innocent question posed to unsuspecting retail and restaurant workers. The question "opens them up" to all the shit the customer wants to throw at them, and generally ruins their day because they are thenceforth opened to everyone who comes into the store. Basically, they are bottoms until closing time because they equated themselves mentally with the store, which everybody is coming inside.
Customer: What time are you open till?
Waitress: The restaurant closes at 11.
Customer: You didn't answer my question!
Waitress: I know!