Popups that appear automatically (e.g, after 30 seconds of browsing a page) will almost inevitably be BLOCKED by all modern popup blockers.
So you do *NOT* want to use a popup. Period. You want to use a "popon". That is, a <div> that appears on top of the rest of the page.
Vic's script is wonderful, but it's probably about 10 times more complex than you possibly need.
Big question: You say you are supposed to "collect" the answer(s) from the popup/popon. What are you supposed to do with the collected answers? If you don't use some kind of server-side coding (PHP/ASP/JSP/etc.), then the only place they can "collect" is in the actual user's machine, which is pretty much useless.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.