Uno Reverse Card

This card allows you to take the attack or spell card that has been placed down by your opponent and immediately use it against them.
One catch, you have to say "No U" when you place it, otherwise your opponent can go "No U" instead and take that card and shuffle it into their deck.
But if both of you forget, then you can just leave it out and say "No U" on your next turn. Or the turn after that. Whenever you remember.