I Doubt It — also called Cheat or Bullshit — is a bluffing game where players discard cards face-down while declaring a rank, lying when it suits them.
Deal all cards evenly. The player left of the dealer starts by playing any number of aces face-down, declaring "one ace, two aces," and so on.
Ranks go up in sequence (aces, twos, threes...). You may play cards of the declared rank or lie and play anything. Any player may call "I doubt it!" — the cards are flipped, and the liar takes the whole pile.
The first player to get rid of all their cards wins.
Cheat, Bullshit, and Bluff are the same game.
You take the entire discard pile as a penalty.