What is the "say uno" rule and how does it impact gameplay in the card game Uno?

The "say uno" rule in Uno requires players to announce "uno" when they have one card left in their hand. If a player forgets to say "uno" and another player catches them before the next player takes their turn, the forgetful player must draw two additional cards as a penalty. This rule adds a strategic element to the game as players must pay attention to their opponents' hands and timing their "uno" announcement to avoid penalties.