What causes my eyes to change colors?

In school and some other places in public, my eyes go from blue-gray to yellow-light gray. I dont understand it! As soon as I get home from school my eyes change back to the normal color. Any ideas why this happens?
Update: Lol I wish I were a vampire! But sadly, I am no Rosalie ;[
Update 2: Yeah i LOVE the color my eyes turn. It looks really cool.
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