Impatient or not, Eren watches Armin closely as he guesses at the purpose of the exercise. Armin's mind is an analytical thing, something hot-blooded Eren has trouble understanding, but something he's honestly admired since they were kids. That's why he needs Armin around sometimes. To make up for the places in which he falls short.

There's a look of distant thought on his face as he turns the concept over in his mind—thought processes and drift compatibility. He wouldn't mind Armin as a partner, they've been inseparable since they were kids but there's some part of him that worries that Armin isn't cut out for this sort of thing—until Armin once again refocuses on the task at hand.

"Why not?" As if this is suddenly important. He's changed gears within seconds, from skeptical to cooperative on the subject of the questioning, a rapid three-sixty turn facilitated largely by Armin's patient explanations. Years of friendship or no, Armin's answer surprises him. Not that he'd ever thought too hard on the matter, but it wouldn't have been his first guess. (Maybe because the first red thing that springs to mind is sentimental. Mikasa and the scarf he'd given her, years and years ago. He doesn't voice it.)