If memory serves, there is a Heinlein book in which one character explains to another that if the natives eating soup with a fork(?) visitors should also eat soup the same way. Does anyone recall the book and the text more exactly?

It was 'pie with a fork', not soup. It's in Space Cadet.
I am in the process of reading it for the umptieth time since I first read it in 1953-4.

He made the same point somewhere (TEFL?) by saying if the natives rub blue mud in their navels, then rub blue mud in your navel, and _Double Star_ he talked about a politician who normally used "formal" table manners would (quite properly) eat corn on the cob with his hands while campaigning out in the sticks.