Twenty questions is a game where one person things of something secret and the other person has twenty yes-or-no questions to narrow down who it can be and finally guess it. “Is the person alive today?” “Yes.” That sort of thing.
The way we play it these days, it’s almost more like a puzzle or co-op experience because of the first rule, which is that the person who came up with the secret should aim for something that will be discovered by question number 17 or thereabouts. If they find the secret a lot sooner, you came up with a too easy secret, and if they don’t find the answer within 20 questions, you came up with a too difficult secret. For that reason, you obviously should pick something that you believe they are aware of.
Some more rules:
The secret can be anything. Luke Skywalker, a dentist, running, the color blue, the smell of a cedarwood pencil, a police car, Marilyn Monroe, feeling sad. “Is it a class (as opposed to an instance)?” can be a good question.
There’s a lot of honor system applications in this game. For example, the secret-keeper cannot change what the secret is and needs to stick to a good poker face throughout the game. If the guessers are kibbitzing and are saying things that indicate they’re on a trail that’s particularly close or particularly far off, let them. (You can write down the secret if you want to make sure the secret-keeper is playing this part honestly.)
The guessers need to make an honest, good-faith effort to only ask questions that they believe will be answered by either a yes or a no. If they fail in doing that, like if they ask “What year was the person born?”, call them out on it with a “that’s not a yes or no question”. But if they do ask something that it seems like they genuinely believe is a yes or no question, the secret-keeper should not be limited to answering yes or no. If your best good-faith answer is “I believe so” or “You can sorta say that” or “I don’t know”, then that’s what you must answer. These “borderland answers” give a lot more clues to the players; they are not allowed to “fish for” these kinds of answers, they should be surprised when they accidentally get one of these but must instead make a good-faith effort to ask binary yes-or-no questions.
For multiplayer (one secret-keeper, multiple guessers) we’ve tried alternating (everyone takes turns asking their question), we’ve tried cannon-fire (everyone can ask a question as soon as they’ve got one), and we’ve tried having one caller: the players are free to say anything (“ask her if it’s something that currently exisist on planet Earth!”) but the secret-keeper only answers when “the caller” officially asks the question (and then you let different people be “the caller” different rounds). I’ve found that turn-taking can lead to a kind of dull game where everyone’s secretly pondering the answer and keeping their best ideas sorta close to the vest, while having a caller leads to more of a curious and open discussion, which is what I prefer. However, turn-taking might be better if you have a large group with some shy participants who often get over-shouted.