B.S., Bull, or the well-known expletive for which they stand, is a card game of unadulterated bluffing. There are only two important factors in the B.S. card game: knowing how to bluff and knowing when to call a bluff. The game is not as sophisticated as, say, poker, but it is a good way to work on your poker face and find out how good your friends are at catching you in a lie.
Get at least three people together and deal out all of the cards between them. Depending on how many people you have, you might have a few extra cards. Although some of your players will have one more card than the rest of the group, it shouldn't make a difference in the outcome of the game.
The player with the two of clubs plays the first card. He calls out a number of twos from one to four, and places that number facedown in the middle. The next player calls out a number of threes and places those cards face down, and so on. After a player plays kings, the next player calls out aces. Play continues in a circle until someone has used up all of his cards. The first player to get rid of his cards wins.
If you think another player is lying about her cards, you have the option of calling out B.S., bull, or the other term–depending on whether or not there are children present. You must call before the next person plays. If you call BS, the player picks up the cards he played from the card pile in the center of the table and turns them over. If he was lying, he has to take the entire deck and add it to his hand. If he was telling the truth, however, you have to take all of the cards from the center and add them to your hand.