"It appears we're at an impasse," Cersei says to Tyrion. "I can't compete with you physically, and you're no match for my brains."
"You're that smart?" he asks jovially.
"Ever hear of Varys? Littlefinger? Maester Luwin?" she says.
"Really?" Tyrion replies. "In that case, I challenge you to a battle of wits."
Tyrion proceeds to fill two goblets with wine, and then turns his back to his sister. When he returns, he places one in front of Cersei and one in front of himself.
"In one of these glasses, I have placed an odorless poison called briocane," he says. "The battle of wits has begun. It ends when you decide and we both drink, and find out who is right and who is dead."
"But it's so simple," Cersei says. "All I have to do is divine from what I know of you. Are you the sort of imp who would put the poison into his own goblet, or his sister's? Now, a clever man would put the poison into his own goblet, because he would know that only a great fool would reach for what she was given. I'm not a great fool, so I can clearly not choose the wine in front of you. But you must have known I was not a great fool; you would have counted on it, so I can clearly not choose the wine in front of me."
"You've made your decision then?" Tyrion asks.
"Not remotely," she replies. "Because briocane comes from Braavos, as everyone knows. And Braavos is entirely peopled with criminals. And criminals are used to having people not trust them, as you are not trusted by me. So I can clearly not choose the wine in front of you."
"Truly, you have a dizzying intellect," Tyrion says.
"Wait till I get going!" Cersei exclaims. "Where was I?"
"Braavos," Tyrion replies.
"Yes -- Braavos, and you must have suspected I would have known the powder's origin, so I can clearly not choose the wine in front of me."
"You're just stalling now."
"You'd like to think that, wouldn't you?" Cersei says, getting even more excited. "You survived my assassin at the battle for King's Landing, which means you're exceptionally strong. So, you could have put the poison in your own goblet, trusting on your strength to save you. So I can clearly not choose the wine in front of you. But, you've also convinced the Stone Crows to follow you without being one of their number, which means you must have studied. And in studying, you must have learned that man is mortal so you would have put the poison as far from yourself as possible, so I can clearly not choose the wine in front of me."
"You're trying to trick me into giving away something," Tyrion says dryly. "It won't work."
"It has worked!" Cersei replies. "You've given everything away -- I know where the poison is."
"Then make your choice," Tyrion says.
"I will. I choose -- what in the world can that be?"
Tyrion turns around to see what she was pointing at. There's nothing there. Cersei, meanwhile, has switched the goblets in secret and is now laughing.
"What's so funny?" Tyrion asks.
"I'll tell you in a minute," she replies. "First, let's drink. You from your glass, and me from mine."
They both drink their wine.
"You guessed wrong, sister," Tyrion says.
"You only think I guessed wrong -- that's what's so funny!" Cersei exclaims. "I switched glasses when your back was turned. You fool."
"You did?" Tyrion asks. "Oh, crap. I can't believe that actually worked."
Tyrion falls dead.
A perfect, highly original ending, you think. But you still need an epilogue.
Your decisions leading to this conclusion have left you with the following options.