Sevens swap hands
Playing a 7 obliges the player who played it to trade their whole hand with one opponent of their choice — it is not optional, so a seven is a card you think twice about laying. Nothing else about the card changes, and zeros are unaffected. Mattel publishes the pair together as Seven-O UNO, its house-rules runner-up, where a zero also passes every hand one seat along; this entry takes the seven alone. Someone who has worked down to two cards can be handed a fistful, so whoever is quietly running away with it becomes the obvious target.
