For now if you want to hide your punchline or joke, you need to just post it as an update, and not in an actual Topic. This is how:

Text between the spoiler tags will be hidden until a button is clicked. [ spoiler intro="Answer" ] Type the answer here!! [ /spoiler] with no spaces after the [ it will look like this
[spoiler intro="Answer" ] Type the answer here!! [/spoiler]