Placing hidden textbox on web page

Question

I want to create a registration form for my project where, when a user has to fill in the fields for registration. One of the field is of "SECURITY QUESTION" where the user is provided with some questions from DropDown Box but if the user clicks on "SPECIFY YOURSELF" option a "TEXTBOX" should appear right below the the drop down box for the user to enter this manual question. The sample of such thing you can see when registering for a yahoo id. So can anyone tell me how can I do that? Any help would me very much appreciated.. Thank You in ADVANCE>