Free WordPress BBSpoiler pluginFree
You can use this plugin to hide part of the text of a post in a nicely-formatted container that will becomes unhidden when clicked on. The plugin can be useful for creating FAQ pages, hiding large pictures, and things like that.
The plugin creates its own “Spoiler” button in the visual editor, but you can also add spoilers directly using tags. For example:
[spoiler title='Title']Spoiler content[/spoiler]
[spoiler title='Title' collapse_link='no']Spoiler content[/spoiler]
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Source from: WordPress.org