Contributed by Johan Wästlund (firstname.lastname@example.org)
Here are the rules of the game played by two players, but with obvious changes (like decreasing the number seven to something smaller) it can be played by any reasonable number of players.
The deck is shuffled, and placed face down between the players. A player on turn draws a card from the deck, looks at it, and chooses between two options: He may discard it, by putting it aside face up, or he may claim it to be a spade. When a card is claimed to be a spade, it is placed face down in front of the player. The cards claimed in this way should form a row, partly covering each other, so that it is clear to the opponent both how many cards that have been claimed so far, and which of them was the last to be claimed.
A player may, at his turn, choose to challenge the last card claimed by his opponent. He does this by turning his opponent's card face up. If it is not a spade, the challenger has won the game, while if it is a spade, the player who claimed it has won. Hence a challenge always decides the game in one way or the other.
If no card is challenged, the player who first claims seven cards without being challenged has won the game. As a consequence, if a player would claim his seventh card in the normal way, his opponent should always challenge it. Therefore, a player who claims a seventh card should never bluff. Consequently, when a player claims his seventh card, he should do this by putting it face up on top of the first six cards claimed, and it must be a spade.
Note that one may only challenge the last card that the opponent has claimed, not any of the previous ones. Since it is not reasonable to discard a spade, the game will always come to an end before the deck has been exhausted (and well before that unless the players are bluffing too little).