Is it against the rules to hide your cards in UNO?

Except the Unorule(You should callUno when you have only one card left),you needn’t to show how many cards you have. However,you can’t hide you card,or misleading others about the number of the cards you have.

Do you have to show your hand in UNO?

There is no rule stating how a player must hold their hand. The only relevant section, is that you must declare UNO when you are down to 1 card, and that if you are caught – you must draw 4 cards. As such, it is “legal” to hide the cards.

Can you put a pickup 4 on a pickup 4 in UNO?

No, it’s not valid to do so since according to the Mattel’s UNO rules the next player forfeit their turn and MUST draw 4 cards from the pile. But for the player after the next player (who had to pick up the 4 cards) it’s a valid play.

What if you say Uno at the same time?

It’s not possible for 2 players to say Uno at the same time. A player has to say it on their turn, before the next player takes their turn. The only time they are punished is for NOT saying “UNO” on their turn before playing their next-to-last card, and only if someone points it out before the next person plays.