If they are not caught saying “Uno” by another player before the next player has taken their turn. If the people want my sources, I'll share them gladly.

Poll below if this becomes popular enough because I was done the injustice of being told to draw two cards after the next person had already done their turn. Yes uno ruins families worse than monopoly.

1. You can't put anything on top of a +2 or +4

2. You can put +2 on +2 and +4 on +4, but you can't put them on each other

UNO Rules - How to Play UNO Card Game