I can't find any confirmation but I'm pretty cure 20q.net is NOT a nueral net. There is no need for it to be. Basicaly it breaks down to this. You have a set of objects and a set of questions. For each object you have 1 or more questions (more = better) that you know the answers to. Start with a random question (hopefully one that divides the rest of your items in half). Then continue to ask questions that divide the rest of your items in half (half say yes and half say no). You should reach the bottum of your tree and be left with the item that the user is guessing. There are some finer points since some items wont have answers to all questions. Obviously though the more questions and items/answers you have the smarter your system seems. I've programmed one of these in the past and they are quite fun and seem quite intellegent.