Difference b/w Troll & Giant?
Okay, I feel like a troll asking this but: What is the difference between a Troll and a Giant? And why did I just compare myself to a troll rather than a giant? There's obviously something to do with the brain. Hmm
Trolls in folklore were not necessarily gigantic in stature, and were more likely to have magical powers, live underground or underwater, and had various other magical traits. However, Rowling seems to have patterned her trolls after modern fantasy literature, in which they are basically just small giants. So the difference seems to be that trolls and giants are both big, ugly, and stupid, but trolls are smaller, uglier, and stupider. We also know that giants are almost extinct, and that trolls can be trained.
Whether trolls are actually related to giants, or a completely separate species, isn't something Rowling has addressed, so you could probably take either view.
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According to Fantastic Beasts and Where to Find Them, trolls are beasts, whereas giants are classified as beings. From Ootp, we know from Hagrid that giants are social and intelligent beings, though quite violent.
Whereas giants are like very large humans (but with proportionally larger heads), trolls are only humanoid in form. Their skin can be green or purple depending on species, and some have horns. They are also exceptionally stupid, and this might have something to do with the fact that their heads (and thus brains) are proportionally smaller than a human's. Also, note that trolls are only twelve feet tall and weigh a tonne, but giants are much larger.
I think it can safely be assumed that giants and trolls are completely different species in the Harry Potter world. Giants can interbreed with humans and are intelligent enough to be classified as beings. Trolls are incredibly stupid (thus the OWL grade, 'T') and look quite different.
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Another thing is that they seem to have a whole different capacity for learning language. Both have to be taught by a human, but giants seem capable of becoming fluent speakers, where as trolls seem capable of only being able to learn a few words, at most.
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Trolls don't seem to have the sort of culture - the way they live, and some of them show real interest in wizards and are fascinated by magic- , and some hierarchy within their society (the gurg, for example), while I think that Trolls base their hierarchy merely on who's got the largest club (they're seen comparing sizes in PoA).
I think Inverarity summed it up quite nicely - Trolls are smaller and especially stupider. And they have less hair. And they smell.
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