A simple way would be to hide the popup in a separate div, or span, with a class to hide the content.
When you click on the item, you would then change the class to display the popup using javascript.

So you need to create a separate div for the popup, a style for the popup to have and an onclick() event for when the text is clicked, we need to show the popup.

(I can't post html code in the code format because the page won't let me)
HTML

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CSS

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JAVASCRIPT
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There's functional stuff for displaying the popup, it's up to you to figure out how to hide/format the popup - I won't do all the work for you