The directive that crosses the boundary

Since server rendering is the default, there has to be a way to say "this part is different". That way is a directive: a bare string literal on the first line of a file, a pattern JavaScript has used before for "use strict". There are two such directives here, and they point in opposite directions, which is why they are so easy to confuse. One marks code that must be sent to the browser. The other marks a function that stays on the server but may be *called* from the browser. That is a server actionA function marked 'use server' that the browser can call directly, but which executes on the server., and it is how submitAnswer grades your answer without exposing the expected answer. A directive marks a boundary, not merely a file: anything imported by a client component is pulled into the browser bundle too. Keeping that boundary small is why answer-form.tsx contains as little as it does.