How do 20 questions AI algorithms work?

Question

Simple online games of 20 questions powered by an eerily accurate AI.

How do they guess so well?

Answer 1

It bills itself as "the neural net on the internet", and therein lies the key. It likely stores the question/answer probabilities in a spare matrix. Using those probabilities, it's able to use a decision tree algorithm to deduce which question to ask that would best narrow down the next question. Once it narrows the number of possible answers to a few dozen, or if it's reached 20 questions already, then it starts reading off the most likely.

The really intriguing aspect of 20q.net is that unlike most decision tree and neural network algorithms I'm aware of, 20q supports a sparse matrix and incremental updates.

Edit: Turns out the answer's been on the net this whole time. Robin Burgener, the inventor, described his algorithm in detail in his 2005 patent filing.