Think of it this way. At every stage, you look at all the knowledge you have, combined with the previous questions and answers you've received, and generate some question. Then you get one of exactly two answers. So you can reach at most 2²⁰ different results, no more.

Even if you allow yourself to use some (presumably weighted) randomness to generate your questions, the expected number of correct guesses is (at most) 1,000,000.

But I don't think anybody really believes this line of reasoning. What's wrong, then? Perhaps we adjust ourselves according to what we know of the person who chose the object, or maybe we "read" them for more information. Odd.

A varitation for 20 questions

So how do you play?

Rule:

"I will always make reference to something that is spherical (basket ball, soccer ball, a planet, a jawbreaker, a pearl, peppercorn, a bubble, a marble)".

question : What is your favorite book?

answer : Around the World in 80 days

question : When trumpets sound how do you feel?

answer : Like putting pearls in my ears

question : What would you do if someone slapped your mom?

answer : Spit out my jawbreaker and bust their head open.

1. Nature : Material / immaterial
2. Nature : Living / Inanimate
3. Nature : Natural / Artificial

20 Questions is a board game by University Games. This 2000 game consists of 400 playing cards with `something', `someone' or `somewhere', for instance: tango, Rembrandt van Rijn or Georgia. Each term goes with 20 indications, with which the players should be able to guess the subject. Georgia will go with descriptions like `I once hosted the Olympics' and `The Black Sea lies at my feet'.

If it's your turn, you take a card from the deck and read out if the subject is a `something', `someone' or `somewhere'. The person to the left of you starts with calling out a number between 1 and 20, which indicates the direction on your card. The player then gets to guess what it is. When the answer is wrong (which is probable in the first few turns), the next player asks for a number to be read and subsequently tries to guess the subject.

This goes on until someone gives the correct answer. Then the reader gets a number of points equal to the number of inquiries, and the one who guessed correctly gets the surplus from 20. For example: `Gouda cheese' has been guessed after 5 turns. The reader earns 5 points, the winning candidate gets 15. This indeed means that the sooner you guess correctly, the more points you earn. The points are cashed by moving your game piece.

Additional points can be earned when you land on a special spot on the board. Here you can win 2 to 10 points. You get a new card with a new subject. The answer should be deduced within 5 descriptive indications, earning you more points if you guess the right answer correctly soon.

Some numbers on the card do not contain directions, but assignments. Instead of getting wiser on the subject, you for instance have to swap places with someone or go back or forth x positions. This can be very frustrating if you're far ahead of the rest, but on the other hand it provided the weaker players with a chance to win too.

The game was first issued in 1991 under the name Quizt't (which also means, "I knew it" in Dutch if you say it swiftly) by entertainment company MB. In the same year it was nominated Game of The Year in Germany thanks to its communicative character. MB reissued the game in 1994 under the title Querdenker (German for something like Question Thinker). In these two versions, there was still a category `sometime', which also featured a year, for instance 1848, or 1972. Because it required certified historians to avoid this category being more like a gamble, it was dropped from the game. It can be played with 2 to 12 players. The more participants, the more exciting the game gets. Why? Because then you'll have to wait longer before it's your turn - and the most thrilling part of the game being the fact that you suddenly realize you know the answer while it's someone else's turn. Patience is a welcome quality in 20 Questions.