Sometimes, a particular answer on a form will trigger follow-up questions. You can, of course, put all of the possible questions on the form and just give directions like "if yes, skip to question 17," but wouldn't it be cool if the follow-up questions appear/disappear depending on the earlier questions.
Here's an example of such a question, based loosely on something I saw on Facebook.com. To see the followup question, choose "in a class" from the pull-down menu.
The implementation is surprisingly easy, and you can see it by viewing the source for this page. The things to know are: