He listens quietly as Katniss lists off the facts - at least, what they've been allowed to know so far. There were deeper questions underlying this situation though that they had to find out as soon as possible, and most of them in his head began with Why...

"Thirteen exists," he added, which was still a shocking realization to himself. "We need to find out more. Maybe if we can find Haymitch?" Surely he wouldn't leave them in the dark forever.