{spoiler}
This is what is visible when the spoiler button is pressed.
This may contain HTML also. So you could have a hidden image.
{/spoiler}

Using double brackets in these samples would make things easier. You can make it look exactly the way it should be by typing [[spoiler]] stuff [[/spoiler]] instead of sticking in extra characters or spaces, or using the wrong characters.

To clarify, Spoilers are made using this code:

[spoiler]
This is the text you want to hide inside the spoiler.
[/spoiler]