Hanabi is a cooperative firework-themed board game in which players collectively try to build five piles of cards representing five different colors of fireworks. Each pile has to be all the same color, and has to start from 1 and work up to 5. The twist is that each player holds their cards facing away from them so that every other player knows what they have, except the player holding those cards. Turns consist of trying to play a card, discarding a card, or giving a hint to another player about their cards in a specified format (e.g. “Here are all your 3s” or “Here are all your green cards”).
While I haven’t played this game nearly as much as the folks over at Hanabi Live, I’ve played it enough for the game to teach me a few things. Because the game is cooperative, it can be fairly easy to “cheat”, by providing your teammates with information that isn’t strictly contained within the specified format of hints, even when you are trying not to do this. Here’s a few examples:
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A player who is new to the game looks around at the cards and asks “What’s the other color besides red, green, yellow, and white?” I can’t see any blue cards.
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After Player A draws a new card, Players B & C stare at it for an unusually long time. The card that you just drew is important in some way. Either it’s a card that you will need to play eventually, or it makes it harder to give you a clue that had been planned.
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After Player A draws a new card, Player B glances at it, then immediately glances at Player C’s hand. The card that you just drew is related to something in Player C’s hand.
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Player A correctly plays a card to the green pile without first checking to make sure the card is green. I not only knew this card was playable, I was also able to deduce that it was green from what I could see.
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Player A reacts positively when Player B plays a card that Player B hadn’t been totally confident about. Possibly: There were two ways I thought you might have interpreted the clue I gave you, so I’m glad you chose that interpretation.
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A player acts surprised after playing a card and realizing it matches the card they think the next player is about to play. Possibly: I think a mistake is about to be made. You should not play what you were planning to.
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Player A gives a clue of the form “Here are all your 3s” to Player B, but accidentally skips one of the threes. Player C points out the mistake. The three that got skipped is not as important right now as the other threes.
Effectively, if you want to make sure that the only information you give is contained within the clues, then absolutely everything else has to be completely consistent. No reactions (positive or negative) to anything anyone else does in the form of speech, facial expressions, eye movements, etc. No commentary related to the game either. Even if you try to do all this, some things just can’t be avoided if you’re playing in person: how long you spend deciding on an action won’t always be the same; you have to look at your teammates’ cards to know what they are; etc.
Hanabi demonstrates the difficulty of secret-keeping. If possible, providing no information is the best way to avoid accidentally providing information that you don’t mean to, at least in situations where providing no information isn’t inherently suspicious.