Since his attention has been drawn to Arya, Tywin asks her where she's from. Her answer is "Maidenpool," which we can safely assume is some non-Winterfell place. Tywin doesn't buy it, so he asks who's the Lord there. Ah! This calls back to the scene in the first season where Arya wasn't paying attention when learning all the Houses. I bet if someone had explained to her that she could use this information while going incognito in the service of her family's enemies, she'd have been more into it. Having said all that, Arya knows the answer is "House Mooton," which I initially heard as "House Mitten." Tywin would like to know what their sigil is, and Arya's storehouse of Maidenpool knowledge has dried up. Tywin tells her it's a red sun, and that he knows she's a Northerner. So she tries again, this time claiming to be from Barrowton. And to prove it, she rattles off the facts: House Dustin, and the sigil is two crossed long-axes beneath a black crown. Tywin appears to accept that, although it didn't sound very plausible to me. He asks what people in the North say about Robb Stark, and Arya says that Robb rides into battle on the back of a giant dire wolf. Also he can turn into a dire wolf and can't be killed. Tywin asks if she believe that. Arya: "No, my lord. Anyone can be killed." And she says that while looking Tywin right in the eyes. They lock eyes for a while, and Tywin finally tells her, "Fetch that water." Tywin's pretty awesome. He'd rather have a prisoner-turned-water-bearer threaten to kill him than a cousin whine about needing to get some sleep in the middle of a war council.