1. To start: one person can think of an object.
2. The other person can ask "yes-no" questions to guess what it is. (That means that the answer to the question is either yes, or no.)
3. Up to 20 questions can be asked (one at a time), and the person with the object in mind has to answer each question truthfully by saying "yes", or "no".
4. But it is good to add one possible answer: irrelevant. (This answer is to be given when the answer is neither "yes" or "no" for the question.)
5. When the object is successfully guessed, or when 20 questions have been asked, switch (a different person becomes the one who thinks of another object to be guessed).