Slightly longer one with a even more difficult prompt...
The knights looked at each other confused for a moment, before movement form the corner of their eyes catches their attention. The rebel of the group, Gwain had surprised everyone by suddenly starting to strip.
Sir Leon's eyebrows rose as he questioned "What are-" But cut off by a swift kick from Sir Elyon who mimed to be quite. Gwain had also turned around to shush Leon, finding the situation far too funny to ruin.
Finishing stripping his chainmail and undershirt, leaving behind his bare upper-half, Gwain quietly and carefully, to the utter surprise of the others, started towards the balcony where the king was located. Keeping his eyes on Arthur, Gwain was unable to keep the grin off of his face. He then shifted his body to turn and rest his bare back against the wall of the balcony, the resulting position was side by side next to Arthur.
The knights, for the most part, at this point were attempting to muffle their sniggers as Gwain lifted his head, hair already wet and hanging limp, remarkably like that of a wet dogs, and proceeded to smirk at the others. Arthur chose this time to start talking, the Knights' appearance having only amounted to a small lapse of time. With a smug smile plastered on his face Arthur said "See I told you merlin-" another round of suppressed sniggers, "- the best way to find relief-" and hands clasped over gawping mouths, "-in a heat wave is standing in the rain." He then lifted his right hand, finger pointing up in an I-told-you-so move, "And since little princess doesn't want her clothing wet, you simply remove it." he said in a mocking voice. "That is unless you're afraid all that time in the tavern has ruined your 'lean physique'." Arthur continued.
Merlin had entered the chambers with a handful of linens, before Arthurs mocking, humming a tune as he went along. However he was abruptly startled by a gloved had wrapping around his mouth. Dropping his load in surprise merlin instantly grabbed at the hand covering his face and almost let some magic loose if not for the owner coming into view. Elyon brought up his other hand in a shushing movement before pointing out to the balcony where Arthur was still contently jabbering on to Gwain.
Merlin's relief quickly switched to shock at the sight before him. Sir Percival motioned to Elyon, pointing out the obvious, "I think he's in shock!" he whispered with amusement. "Give it a minute." came the reply, and as if on queue Merlin's face transformed into a mirror of the knights mere moments before.
The amusement that Merlin felt in that moment was unmissable to everyone, his smile so wide it took over his face and his eyes crinkled. Gwain, closer to the doors than Arthur caught a glance at the disruption and, like before, lifted his head to spot merlin, his grin just as large. Returning to Arthurs one-sided conversation, Gwain decided to push the situation further. Shuffling closer to Arthur so that their arms were brushing against each other, Gwain commenced his plan. Arthur who, because of the rain, had kept his eyes closed, frowned before shifting uneasily. The Knights (plus merlin) found further amusement in the clearly uncomfortable Arthur and before anything could be said Gwain pounced with puckered lips.
Arthur screamed.
"MERLIN!" and proceeded to desperately scramble away.
Then seeing Gawain on the floor beating his fists in fits of laughter, and hearing of the knights amusement. Standing shocked in his room-his own room, in his own castle, in his own kingdom! Arthur was slow to respond. Merlin couldn't resist adding in..."I know Gawain's a charmer but didn't think he could charm you out of your shirt too!"