"I wouldn't do that if I were you…" Eren's cryptic warning was almost lost among the constant rustling of leaves.

Annie's hand instinctually paused on its way to her ring. "And what, exactly, is stopping me?" she asked challengingly, even though she was mentally begging him to stand down.

"You would almost certainly, probably, definitely kill me if you transformed," Eren replied simply.

She, along with Bertholdt and Reiner, just stared at him for a long second before she finally spoke up. "I ripped you out of your titan without killing you, remember? I'm pretty sure that gentleness won't be an issue."

"You're missing the point…" Eren said cryptically before switching back to his normal tone. "It's simple physics and chemistry really. You see, the law of conservation of mass basically states that matter cannot be created nor destroyed. If you want to give something more mass, you'll have to take it from something else. If you want to remove mass from something, you've got to put the unwanted mass somewhere else. In essence, something can't come from nothing; nothing comes from nothing."

"Is there a point to all of this or are you just stalling for time?"

"The point is, in order for you to go from a 150-centimeter, 54-ish kilogram…?" He looked to her for confirmation.

"Something like that," she said airily.

"…human to a 14-meter, 496-kilogram titan, you're going to have to pull 442 kilograms worth of stuff from anywhere you can. Grass, trees, buildings, titans, me."

"Wait, what?"

"I'll get to that. Now, I'm gonna fly through this next part really quickly because you're all probably going to be bored as hell if I take my time so here… we… go!"

Eren took a deep breath.

"By piecing together memories of chemical analysis research conducted during my nerd phase, please don't ask, I can tell you, off the top of my head, that the human body is composed of 65% oxygen, 18.5% carbon, 9.5% hydrogen, 3.3% nitrogen, 1.5% calcium, 1% phosphorus, 0.4% potassium, 0.3% sulfur, 0.2% sodium, 0.2% chlorine, and 0.1% magnesium with iron, fluorine, zinc, silicon, rubidium, strontium, bromine, lead, copper, aluminum, cadmium, cerium, barium, iodine, tin, titanium, boron, nickel, selenium, chromium, manganese, arsenic, lithium, cesium, mercury, germanium, molybdenum, cobalt, antimony, silver, niobium, zirconium, lanthanium, gallium, tellurium, yttrium, bismuth, thallium, indium, gold, scandium, tantalum, vanadium, thorium, uranium, samarium, beryllium, and tungsten making up the remaining less than 0.1% of the 'stuff' you will need to pull in."

Eren was gasping for air when he finished before pumping his fist. "Hell yeah! First try!"

The three Warrior's faces showed varying degrees of shock, horror, and a little bit of awe. Reiner spoke up first, "Are… are you okay? Do you need a minute to catch your breath?"

"I'm good," he assured them.

"Are you sure?" Bertholdt checked.

Eren paused for a second, like he was taking mental inventory. "Yep. Yeah, I'm good."

"Okay," Bertholdt replied dubiously.

"Anyway," Eren continued, "most of these are in the world in abundance. Like, seriously, you can find carbon, sulfur, calcium, potassium, sodium, magnesium, and iron in dirt alone. The real issues begin with oxygen and hydrogen, which is unfortunate because those are two of the big three."

"Wait, pause," Annie held a hand out. "Isn't there oxygen in the air? And what about water? Why are those the most difficult to obtain?"

"Actually, the air we breathe is mostly nitrogen. Less than a quarter of it is oxygen and at sea level, there's only a quarter of a kilogram of oxygen per cubic meter. Keep in mind that we'd need to stuff you with 287.3 kilograms of the stuff. That means you're going to be taking oxygen from everything and everyone within 1,149.2 cubic meters of yourself."

Annie paled slightly. "That's… horrifying."

"Just wait until I tell you about hydrogen!" Eren replied cheerfully.

"I don't suppose water vapor would suffice?" Bertholdt asked.

"Yeah, no. Even on the most humid of days, which it is not, you're looking at 200 picograms per cubic kilometer. That means, in order to get enough hydrogen to build this fucking mid-sized Titan, I say again, mid-sized Titan, you're going to have to pull hydrogen from every available nearby source. And nearby is relative. Any and all hydrogen sources within 223 billion kilometers of you is fair game. And since there is very, very little hydrogen in open air, that means you'd better pick a god and pray that they put a barrel of water between us when you decide to transform or guess what? Reiner, Bertholdt, and I are the most water-rich sources available and you're going to squeeze us dry like sponges." Eren stared the three of them down, smirking. "There'd be no use bringing me back dead, right? I'm guessing if you could, you would have just eaten me and puked me back up when you got to your destination."

"I'd be proud of you if this were literally any other situation," Annie admitted. Reiner nodded, while Bertholdt looked like he was going to be sick. But Eren was on a roll.

"And Annie's just an extreme example, believe it or not. Anything undergoing an extreme growth spurt in fractions of a second would be like a goddamn hand grenade going off as it immediately sucks in all available nutrients and elements in the vicinity, bursting pipes in local buildings to get their water, killing other humans and titans in the area as it robs them of their vital vitamins and minerals, bursting eardrums from the massive pressure differentials caused by huge volumes of the local atmosphere immediately collapsing,blasting holes in the ground six meters deep and sending all rejected elements flying out in all directions at high fractions of the speed of light, IONIZING NEARBY ATOMS AND SENDING GAMMA RADIATION AND FLYING DEBRIS INTO ANYONE FOOLISH ENOUGH TO NOT CLEAR THE AREA AS QUICKLY AS POSSIBLE! The world should be thankful that there are only four titan shifters currently active or else it would look less like a green and blue marble in space and more like a nightmare beyond the horrors of hell, filled with irradiated craters, nutrient-thirsty monsters, and idiot humans who weren't smart enough to murder every shifter in sight with lead pipes after the first one exploded into their next form. Forget international racism and forget painful murder by titans, shifters, by nature, are blackholes of voracious destruction, self-activating time bombs with the faces of children and teenagers, being convinced that they're saving the world while never noticing that they're slowly killing their families, friends, and neighbors. And all with a smile on their faces," Eren took a deep breath, not realizing he'd been pacing until he'd stopped.

He finally looked up to the faces of his kidnappers. Bertholdt had actually passed out, Reiner was absentmindedly tending to him while trying to process everything he'd just heard, and Annie looked like she was in the middle of a full-on crisis as she sat with her back against the tree trunk, staring blankly in the general direction of her boots with her head in her hands. And that was how Levi and the rest of the Survey Corps. found them 20 minutes later.