How come in twilight vampires sparkle?

The simple, straight forward answer is because Stephenie Meyer says they do. As to exactly why, you'd have to ask her, as to my memory she never adequately explains the phenomenon.

However, given other information in the books, we can attempt to extrapolate a "scientifically sound" reason.

She makes mention of how vampire skin is hard and cold, stone like, almost impregnable to harm. When limbs move, skin folds to allow the changing surface area caused by the movement. Skin so hard as to be bulletproof would, by default, be so stiff as to practically prohibit any kind of functional, fluid movement. Unless, of course, the skin was more like overlapping scales. Snakes, sharks, and many other animals have overlapping layers of "armor" or skin and move quite well. So one could argue that when someone changes into a vampire, their skin hardens and becomes more like millions of diamond-like, overlapping scales. The pale skin indicates a lack of pigments, which could allow a semi-translucency that would let sunlight into the scales, where it would be refracted and thus "sparkle" similar to a cut gem.

Mind you, this is all pure theory and I won't promise the science is 100%, but it sounds reasonable on the surface. Or at least reasonable enough to explain a fictional characteristic. =)

I don't know. I was hoping that Edward would look really gross and scary, but instead he sparkled. :( If a creature did sparkle in real life, I would imagine that the purpose would be to either attract mates or prey that is easily distracted by something shiny, like me.

I guess Stephanie Meyer wanted to do something different since she basically started a whole new story of vampires.

~Buffy

Because twilight made vampires into a bunch of teenage, goth pu$$ies. Why? The target market is 15 year old girls

Because the idea behind them being so beautiful and perfect (including their voice, smell etc... including sparkling) is to attract people to them. So that they can kill them and drink the persons blood. It also adds another dimension in the story to explain why they cant go out in sunlight... burning alive is a little dramatic.

Stephenie Meyer made them sparkly based on a dream she had.

because its all based on Stephanie Meyers thoughts and views on how she portrayed Edward, She wanted to show how beautiful and perfect he was!

I don't think there actually was a point. Its just the way S.M made it

the author wanted to do something different, proabably since her vampires are good vampires

because Stephanie Meyer is an idiot !! :)