Well, it's very easy to do using JavaScript.
First of all, click the following link to see an example of our pop-up window:

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Now, let's take a simple HTML anchor tag like the one below:

<A href="#back_to_the_top">

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</A>

and convert it so that it can display a pop-up window when clicked:

<A href="#back_to_the_top" 

OnClick = "hint_wnd = window.open( '', 

'hint_wnd', 'width=50, height=60, resizable=no, scrollbars=no' );

hint_wnd.document.write( 'Hello there!');

return true;"

>

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</A>

As you can see, we simply added an "OnClick" event tag, made it open a window named "hint_wnd," and finally wrote the string "Hello, there!" to the "hint_wnd" window.

Test it!  Back to the top of the page...