A/N: Cheated a bit. This one's 101 words. Don't sue me! :D
He knows Lancelot already knows as soon as the nonplussed knight opens his mouth, and he can't help the small spike of jealousy in his chest.
"You must understand, m'lord, Merlin cannot help what he is. Magic is as much a part of him as blood. He was born with it."
"That isn't possible," Arthur replies, but his knight's look as the other sets down his newly polished boot makes him waver. "Evil is learned."
Lancelot laughs and says, "Arthur, think on it. Could Merlin, Merlin, really be evil?"
Arthur leaves him still laughing, feeling more than a little confused.