How to create a spoiler or show-hide effect

Here is  a very simple show/hide effect code for you to copy and use. This effect is also known as a spoiler or a peek-a-boo.  It uses a small inline Javascript. You can use this code anywhere, be it in the sidebar or in your blog post, as long as you can add the spoiler code to it. [Update Mar 2013] See our new and improved  content spoiler. This code will add a button, clicking it will expand