Stacking is the custom rule where you can play a Draw Two card on a Draw Two card or a Wild Draw Four card on a Wild Draw Four card. This isn't allowed in standard Uno but is a rule that many people use. The winner is the first to reach 50, 100 or 150 points (2, 3 or 4 player game). What's a challenge? A Wild Draw Four card can be "legally" played only if the player holds no other cards of the current colour. If the player is challenged and has played the card illegally, they must pick up four cards. If the challenge fails then the challenger must pick up the four cards in addition to two penalty cards.
