OH I just had a sudden memory. My parents took us to a fancy restaurant, and when my sister looked over her shoulder at something a waiter swooped in from the other side and took away her half-finished food. My sister ran after him and took it back.

When she returned my father explained that it was because she had her silverware in the "I'm done" position.

So that must be where I learned it. Note that it is entirely in character for my parents to not teach us things and then be surprised when we didn't know them.