What the Rule?!

Find the rule! Play your hand cards cleverly to find out which card can be placed according to the secret rule. If you get it right, you will be rewarded with points. Will you be able to uncover the secret rule? Or will you end up thinking, "What the Rule?!".

At the beginning of each round, one player makes up a rule and becomes the game master for that round. The other players play their hand cards in turn. When a card matching the rule is played, the player may try to guess the secret rule. When a player reveals the correct rule, the round ends. The next round begins with a new game master and a new rule. The game is played with 144 playing cards, with each card appearing exactly twice in the game. The cards are distinguished by four different characteristics:

 The shape: "circle", "triangle" or "square".
 The number of shapes: "1", "2", "3", "4", "5" or "6".
 The filling of the shapes: "filled" or "unfilled". 
 The color of the card: "white" or "black".

A simple rule would be, for example: "Always alternate black and white cards". The game ends after each player has been the game master once. The player with the highest score wins.