Secret signals on the job: Waitresses at some 19th-century restaurants ensured speedy drink service by communicating with a non-verbal code. One server took orders, then placed each customer’s cup to indicate exactly what the customer wanted...
The term sun grin literally means a kind of squinting expression caused by facing bright sunlight. Metaphorically, though, it indicates a fixed or humorless grin. This is part of a complete episode.