The shock of having produced an arm almost without thinking was followed shortly by how much everybody changed around him so quickly. The Elite Squad had carried itself as professional, courteous, and even friendly when dealing with him, like he wasn't so much a monster but ust another person with just another set of skills they could add to their own. After the light and the steam, however, Eren witnessed a transformation so swift that all he could do was stare. Levi and Sara were keeping the others from practically pouncing and killing him on the spot, the look in their eyes almost frantic, like they were looking at a threat or an abomination.
Which he was.
The entire time, they had been using their training to keep calm in a threatening situation, when in reality they were ill-at-ease, tense and under pressure as they were to escort a Titan around. They thought they had been mentally prepared for this, but they hadn't, and it showed. Even Kate had flinched at all this, though she didn't remain violent. She was currently glaring at Hange, who was anything BUT freaked out, at this point. No, of course not. She wanted to touch it, and even if Eren had the resolve to say no, she was gonna do it anywway. The result? She drew back her hands in pain, shouting that it was hot without the skin on.
"Well, of course it's hot. You can see the steam rising."
Berratement towards Hange - which she ignored - followed as she rubbed her hands in the dirt, and then proceeded to ask Eren a slew of questions about the arm, what he could feel, if he could make it move, etc. Eren looked down at the thing he was attached to, realized what would happen if he pulled himself free, and did so. Naturally, Hange still wanted to test it first, which left her in a panic, but then she noticed the spoon. All that noise and mess, and yet the Titan arm had performed something so simple, yet requiring actual manual dexterity that no regular Titan had any hopes to perform. She pulled it out - Again, it was hot - and her eyes lit up with many thoughts regarding what this meant as people watched the arm dissolve.
Crisis averted, everyone could calm down...mostly.
"I've got it!"
I mean, there were some things - some people - would would never change. And while the phrase 'Eureka!' was never going to be uttered, due to nobody here knowing the cultural significance, Hange Zoe's declaration was about as close to it as they could possibly get. She had figured it out, and it all lay within the event that occurred just now.
"Titans need the sun's energy to move. They're not nearly as active during the night as day. For Eren to transform, he must have sufficient light to suddenly create that thunder and change his form around him."
This was as good an explanation for why he didn't transform inside the well as any. In fact, they'd practically proven the given limitation in his power by doing so. Likewise, their keeping him in a dark room so that if he transformed, he'd just be stuck where he was prevented him from doing so in the first place. They could try again at night and probably get the same results, and then they'd be positive. But this begged the question for just about anybody or everybody: Why now? To this, Hange simply turned to Eren.
"Eren, what were you doing before this occurred?"
"I was reaching for the spoon I'd dropped."
"Exactly. You were in pain and now you had sunlight, so you needed some kind of clear purpose, and then POW! A giant arm is grasping the spoon. It isn't even bent."
This further proved that not only could they predict and prevent unexpected transformation, but they could reliably know that Eren had actual control, that in fact he needed a reason to, or else it might not happen. So, it was kind of fortunate - to Kate's mind - that there was probably nobody more focused on killing the Titans as Eren Jaeger. It was an obsession, much like her own work, which reminded her...
"Aunt Hange, I think it's time to show them..."
"Show them? Oh, right! Yes, we should."
It was time for the demonstration, though first...Eren would be meeting an unexpected duo.