This is a card game I learned to play with Rook cards, though any cards with multiple sets of 4 numbers may be used.
Number of Players: 4, 6, 8 (only these even numbers)
Objective: Collect 4 cards of the same number each round, signal to your partner your achievement, and be the team with the most points.
Preparation: 4 cards are dealt face-down to each player. Pair off into teams of two players. Each team secretively decides on a subtle sign (flaring nostrils, biting cheeks, holding cards a particular way, etc.) that will be used during the game, with the intent to get their partner's attention but not the attention of the other players.
The Play: The dealer takes four cards from off the top of the remaining deck and puts them face-up in the middle of the table. Players are free to take the cards off the table and exchange them with cards in their hands (in no particular order), so long as each player keeps 4 cards in their hand when they are done. When no players want the cards in the middle, the dealer replaces the cards on the table with 4 new cards from the top of the remaining deck.
Scoring: The goal is to get 4 cards of the same number in your hand and then as subtly as possible make your signal to get your partner to say "Kent." That team then gets a point if no one catches them before "Kent" is said.
If an opposing team catches you making the sign they may say "obvious" and then must identify the sign that was being made. If the accuser is successful at identifying the sign, they get a point, otherwise they lose a point. The team with the discovered sign must regroup and come up with a new sign.