This one is set after my fic, Through Golden Eyes, but if you haven't read that one, all you have to know is that Arthur knows about Merlin's magic at this time. And Merlin is allergic to the flowers on Arthur's table...
Sneeze of a Sorcerer
Merlin did not have even the chance to warn Arthur before it happened. The tickling in his nose grew unbearable, and a sneeze as forceful as a winter wind exploded past his control.
When he opened his eyes again, the room was blanketed in white and Arthur's hair was frozen like blondish-blue spikes straight atop his head.
The prince dropped the spoon held halfway to his mouth, which landed in soup now as hard as rock, and Merlin let out the very first heat spell he could recall. The snow and ice vanished from every corner of the room, leaving Arthur staring, opened-mouthed, at his again-steaming soup.
Merlin innocently rolled his eyes upward, rocking on the balls of his feet for a moment, torn between bursting into laughter, feigning ignorance (which would probably be pointless, since Arthur was well aware of his magic), and assuring his master that no, this did not mean his powers could be dangerous after all.
When he finally grew brave enough to sneak a peek at Arthur's face, their eyes met—tentative gray to astounded blue—and their reaction hit them both like a sudden rainstorm.
Five minutes later, when Uther Pendragon opened the door of his son's rooms, he was unsure whether he should be appalled or concerned to see Arthur half-bent in his chair, tears spilling down his reddened cheeks, with his servant limply collapsed on the floor beside him, both so lost in their wild hysterics neither noticed him at all.
Uther closed the door and left them there, wondering, not for the first time, if perhaps it had been a mistake to put that idiot boy in the position of Arthur's servant.
Inside the still-chilled room, Merlin had never laughed so hard in his life...or been so happy.