Its own deck of colours and actions, and one word to say at one card.
Played with its own deck rather than a 52-card one, and probably the most widely sold card game in the world. The mechanic is crazy eights; the deck is what makes it.
108 cards in four colours — red, yellow, green and blue. Each colour has one zero, two each of one to nine, and two each of skip, reverse and draw two. There are also four wild cards and four wild draw four cards.
Seven cards each. The rest is the draw pile, and one card is turned face up to start.
Play a card matching the top card by colour, by number, or by symbol. Or play a wild. If you cannot play, draw one card — and you may play it immediately if it fits.
Skip: the next player loses their turn. Reverse: the direction of play turns around. With two players it acts as a skip. Draw two: the next player draws two and loses their turn. Wild: name any colour to continue in. Wild draw four: name a colour and the next player draws four. Officially you may only play it when you hold no card of the current colour, and a suspicious opponent may challenge you — if they are right you draw four instead.
When you play your second-to-last card you must say uno. If another player catches you before the next player takes their turn, you draw two.
The winner scores the cards left in everyone else's hands: number cards face value, action cards 20, wilds 50. First to 500 wins the match.