Spoilers are text which is hidden until you move cursor above it. It might be useful when for separating full solution from a hint.

See also: 

 * [Spoilers](https://mathoverflow.net/editing-help#spoilers) in the Help Center
 * [Add markdown support for hidden-until-you-click text (aka spoilers)](https://meta.stackexchange.com/a/71396) on Meta Stack Exchange