Hidden content replacing previous content

We are building a website for entering a screening questionnaire. This website contains a virtual agent, which responds empathic to your answers. A test-version is online at www <dot> few <dot> vu <dot> nl /ghazanfa/changingmoods5.php (the source code can also be checked from this webpage).

We want to have one question per page, without having to reload the virtual agent each time.

Our idea is to do this by letting each question be hidden content. The next button would then replace the previous question by the next question.

Would anyone know how to do this? We tried to search using google and a search function on several forums, but couldn't find the answer..