10 Things You Didn't Know About Harry Potter's Scar
5. The Scar Only Pains Harry When Triggered By The "Master Soul"
Spoiler alert for all three of you who haven't seen Harry Potter yet, but in The Deathly Hallows, it's revealed that Harry is a Horcrux - a vessel containing a piece of Lord Voldemort's fractured soul.
This is the reason Harry can talk to snakes, and it's also what links his and Voldemort's minds together. But an unfortunate side effect of having this evil live inside his body is that Harry will often feel an immense pain in his forehead, something triggered whenever Voldemort is in the vicinity - or, because of that aforementioned mind-link, whenever ol' snake-nose feels a particularly strong emotion.
Over the years, this explanation has caused some confusion in the Harry Potter fan community. If Harry's scar hurts whenever Voldemort is around, why doesn't it hurt in The Chamber Of Secrets when he faces off against Tom Riddle? Why doesn't it hurt when he's near any Horcrux, the other pieces of Voldemort's soul?
The answer - which seems quite obvious, once you know it - is that Harry will only feel pain when he's triggered by the master soul, the piece that still lives within Voldemort. Since the other Horcruxes (Nagini, the diadem) are not master soul pieces - and Riddle is just a projection in The Chamber Of Secrets - Harry can face them pain-free.