Once you do finish off a Big Daddy you can now adopt a little sister and carry her all over Rapture on your shoulder. Come across a dead corpse oozing with Adam and you can send her off to harvest it. But be warned; doing so will alert all enemies nearby who'll descend on the two of you in a matter of minutes. Of course you can plan in advance and set up a plethora of traps around the area to get the jump on them. Even so these "Gathering" encounters are extremely intense and are an awesome addition to this series.