SPOILER ALERT! The following article contains plot details about book 7. Read at your own risk.
How did Godric Gryffindor’s sword get into the Sorting Hat after Griphook the goblin took it? —Amy
The same way it got from Dumbledore’s office into Harry’s hand in book 2 — because Neville Longbottom is a true (and brave) Gryffindor.
Why was Draco the rightful owner of the Elder Wand? How did that work against Voldemort? —Nick
Deep breath: When Draco disarmed Dumbledore at the end of book 6, the Elder Wand — won by the headmaster when he dueled Grindelwald — passed its allegiance to young Malfoy, not to Snape, whose ”murder” of Dumbledore was prearranged and therefore not a defeat. Then Harry won Draco’s wand at Malfoy Manor. So when Voldemort tried to kill Harry with the Elder Wand and Harry countered using Draco’s wand, the two wands effectively canceled each other out. The Elder Wand realized it was attacking Draco’s wand, refused to act against its master, and rebounded the Killing Curse back onto Voldemort. The Dark Lord never had a chance.
Who was the ugly baby crying in King’s Cross that Dumbledore said couldn’t be helped? —Andrea
The dead portion of Voldemort’s soul that had been a part of Harry.
Why wasn’t Kreacher freed when Harry gave him Regulus’ locket? —Jenny
A locket isn’t clothing.
Students at the new Hogwarts had to prove their blood status to attend. So how did Muggle-born Colin Creevey manage to stay? —Maddie
He didn’t. Colin likely returned for the battle along with other members of Dumbledore’s Army (alerted by their enchanted Galleons).
Voldemort’s kin, the Gaunts, had kept the Resurrection Stone; Harry’s ancestors held the Invisibility Cloak. Since those items’ original owners were brothers, are Voldemort and Harry related? —Sara
Perhaps, but there’s been so much intermarrying among wizard families that we suspect nearly everyone with magical blood is related to some degree.
Harry and Ginny’s kids are all named after figures from their past. So what’s the significance of Ron and Hermione’s kids’ names, Rose and Hugo? —Jen
Beats us. Because they have the same first letters as their parents’ monikers? Or Rowling just liked the names? Or she’s a serious Lost fan?