Officially unknown, but there are three main theories:
1) Gold. Just look at the color.
2) Marcellus Wallace’s soul. Many people point to circumstantial evidence to argue this case.
3) “The stuff dreams are made of.” In other words; it doesn’t matter. – Quentin Tarantino is the one who said this.
4) The same thing that was in the silver case in Ronin.
-why does Mike say he and Sully have known each other since third grade, if they met in college?
Bad research on the part of the writers.
Remember that we are arguing things where the writers/producers purposefully integrated explanations into canon.
-if Boris the Animal erased K/Kay from existance, how come J/Jay is still an agent? For that matter, wouldn't this end up destroying the Earth, since no one would be left to stop Edgar the Bug and thus pacify the Arquillians?
The producers have stated that each film (and the TV series AND each run of comic books) runs in its own continuity, so any discrepancies are independent to each continuity.
-if seeing yourself in another time period causes the universe to be destroyed, how come the universe wasn't destroyed when Marty saw himself escaping from the Lybians? Or when Future Biff met 1955 Biff?
Doc Brown was just theorizing, not stating fact. Also, that was just “a worst case scenario”; the damage could be localized to just our galaxy.
-why didn't General Zod terraform Mars?
Because Zod is a megalomaniacal idiot incapable of simple logic. No, I’m not exaggerating; he’s been that way since the original comics, through the first and second films and in Smallville.
-how come none of the Jedis could read Palpatine's mind and figure out that he was Darth Sidious?
A) Because he is extremely strong in the ways of the force
B) Because he’s very good at masking himself, his emotions and his intentions
C) Because Jedi don’t actually read minds.
-how can Leia remember her real mother if she died when Leia was a baby?
Some people in real life do have some vague impressions of their first few days. Also, Leia is force-sensitive; she might have felt Padme’s emotions before she was even born.
-how come Obi-Wan doesn't realize that "only Siths deal in absolutes" IS an absolute?
He knows; it’s his wry sense of humor coming through.
-why was Lavender Brown black when she originally appeared, but white in all the rest of her appearances?
No explanation needed; this is not really an issue of canon;
This is a very interesting case and besides canon, involves something called “apocrypha”, “semi-canon” and “practical adaptation”: In the Harry Potter fan community, the BOOKS, not the films, are the primary canon; the films are semi-canonical, meaning that so long as it does not contradict the books, it can be considered canon. Tertiary sources, with the exception of “Word of God” statements by JK, are “apocrypha”, meaning that, for purposes of canon, they are a couple of steps above “fanfic”. Ok, those were the basics: the two actresses who play Lavender in the second and third film are identified only in the credits, ‘’not’’ on-camera; this places the identity of the character being played in question. In the books, Lavender is not actually described, so canon is no help; in the films, we have THE definitive Lavender brown being played unequivocally by Jess Cave, since this does NOT contradict the books, she is the canonical Lavender Brown and the other two are just “background students”
BUT there are also the realities of “practical adaptation”, where due to various practical considerations, an actor/actress is unavailable, but the character is too important to drop. Google up “the other Darren”.
-same thing from Harvey Dent. Why was he black before he became Two Face, but white after he became Two Face?
Again, not canon; in the first film he was “always black” and in the second film he was “always white”. Darren syndrome applies.
-what, exactly, was stopping Alice from simply eating another piece of the mushroom again and not waste any time this time?
This goes way beyond discussions on canon.
-how can AURYN affect the real world in part 3 if it only has power in Fantasia?
-if the residents of Fantasia can only exist in Fantasia, how come entering the real world in part 3 didn't kill them?
-how can Bastian's stepsister use AURYN?
Links of blood and pure heart.
-why does the Childlike Empress in part 3 look like she in her 20s?
-why doesn't Gandalf just call on the eagles to dtop the One Ring on Mount Doom?
Because they would’ve been seen by Sauron and Mount Doom would’ve been an impregnable fortress bristling with archers and ballistae.
-how did Shredder survive being crushed by a trash compactor? And if he can survive that, how come a bunch of wood falling on him kills him?
-and why was he THREATENING that girl with the Mutagen? In this universe, the Mutagen turns you big and muscular, so if he did use the Mutagen on the girl, all that would have happened is that the girl would have turned big and muscular and kicked Shredder's ass, how does that help him?
-why didn't Skynet send a Terminator to kill Sarah Connor when she was 3 years old?
They actually explain this one on-screen; all it knew was that John Connor’s mother was named “Sarah Connor” and lived in L.A. before Judgement day; without additional information, it would not know where she lived when she was 3; she could’ve liven in another state, or had a different last name (changing it due to adoption, for example).