Somehow or another, one of those twisted minds eventually realised you could harvest Adam from the dead. Their bright idea was to turn schoolgirls into "Little Sisters". They live in the walls (as all good little girls should) and when people die, they jump out with their syringes, wander over to the corpse and suck the Adam remnants out of them to be re-processed. Of course, they re-process the Adam themselves by ingesting it, which in-turn makes them "near invulnerable to damage" (according to various audio diaries). Unfortunately, it seems nobody told the game designers that because they went ahead and created the Big Daddy. The Big Daddy is a giant guy in a diving suit who wanders around solely to protect the Little Sisters... and those guys are tough as hell.

Bridgette Tenenbaum, the scientist who helped create the girls, also seems to forget that the Little Sisters are invulnerable. In the very first moment you're introduced to them, she's busy saving one of her little girls from imminent death which is funny because no matter what you do, you can't kill the litle girls. Once their Big Daddy is dead, you can "harvest" or "save" them to get Adam (once Tenenbaum throws you the special Plasmid) but you can't shoot them. Apparently, bullets bounce off their immortal child-like hides. Not even the trusty wrench will hit them which is particularly interesting considering the first time you meet one, Tenenbaum is saving a girl who's about to be killed by a guy wielding... a wrench. Apparently he hadn't been told either. Also funny is that Dr Tenenbaum shoots the guy to save the girl but then happily walks off, rather than hanging around to see if you're going to save her or not... That just seems like a half-hearted effort at saving her to me. Tenenbaum will even help you out later in the game, despite the fact you've sucked the life out of every single Little Sister you've met. Like I said, BioShock makes a lot of sense until you start to think about it a little too deeply.​