To show and hide contents in Drupal we can use the javascript library in the core and obtain the same effect that we get by default in the collapsible fields of Drupal forms. All is needed is to invoke the script collapse.js in the body of the content (node or block) we are creating:

<script type="text/javascript" src="/misc/collapse.js"></script>

and insert the collapsible content in a fieldset to which we will add the class collapsible collapsed:

<fieldset class="collapsible collapsed">  
    <legend>Title of collapsible content</legend>  
    <div>Hidden text</div>  

Select Full HTML for the input format and submit your content. The result will be the following: