Here's what I'm looking for:
A small form that asks the visitor for some word or number (the answer to which they get elsewhere - it's part of a riddle kind of game I am having on my site).
User types in the word or number and hits "submit". If it matches the right answer then he is taken to a certain URL (where the game continues). If it's not the right answer then he is taken to some other page where I will tell him to try again.