Collapsing Text Box

I'm trying to create a "teaser" text box that will allow me to do the following

The text box will be a fixed size however the text will be longer then the text box itself and I want to add something like READ MORE and when you click on it, the text box either expands or has a pop-up which will show all of the text and when you are done, it will collapse again.

Does anyone know how to do that?