I don't know of a way for it to be hidden in the feature/code sense, but maybe try having the first line or two be a bunch of --------------------- or something like 'Click for answer/punchline. Not perfect, but it might achieve what you're looking for.
I have been trying to figure out how to make it so when I post, the first line of text isn't visible until someone clicks on the thread.
An example of this being used would be a joke, you don't necessarily want part of your punchline visible without having to click on the post.
I don't know of a way for it to be hidden in the feature/code sense, but maybe try having the first line or two be a bunch of --------------------- or something like 'Click for answer/punchline. Not perfect, but it might achieve what you're looking for.