Summary: Loki's worked out why Odin brought him here. (An AU set in the growing-up-in-Asgard era.)
"I've worked it out," Loki announced, sitting down and swinging his legs over the railing next to Thor.
Thor turned to him with a bright grin, unconsciously loosing the rope in his hands. From below there was a bellow of outrage quickly cut off, as Sif fell the last twenty yards into the icy pool at the foot of the cliffs.
Loki's eyes were dancing. Thor gripped the railing and leaned out, his face whipped by the spray from the falls. Fandral was dragging Sif's lanky twelve-year-old form over the stern of the small boat the three of them had lowered earlier. She toppled in and scrambled to her feet, wobbling, and yelled something up at him that sounded like a demand for an explanation, though it was hard to hear over the falls. Thor gave her a respectfully apologetic salute. He followed it up with a wave to indicate they should go on without him this time.
Loki had been avoiding him lately; this was the first time in months they'd kept company without Thor having to hunt Loki down and drag him out of some bolthole by his elbow.
"What have you worked out, brother?"
Loki's mouth moved, not quite in amusement. "Among other things, why the Allfather keeps telling you not to call me that," he said.
Thor threw his arm around Loki's shoulders, grinning when that caused the two of them to sway perilously out over the falls. "You are my brother in spirit, if not in blood!" he cried, raising his voice over Loki's shouted protest as they swayed towards the drop a second time.
Loki hooked his ankle behind Thor's, sending them both sprawling back onto the smooth rocks. Thor immediately rolled after him, seizing Loki's forearm in a grip that was half immobilisation and half a warrior's embrace. Then he couldn't help his competitive instincts and he started pushing, turning it into an arm wrestle.
Loki bucked, slipping his arm free, and when Thor lunged again the arm he seized was nothing but air and light, and then Loki was popping up on Thor's left, rising smoothly to his feet.
Thor stared, delighted. "That's new," he said. "That trick. When did you learn that?"
Loki adjusted his shirt cuffs. "Last month," he said. "I've been waiting for the right moment to use it." He lifted his head and gave Thor a sudden grin, that rarest of expressions on his face.
Thor got to his feet, dusting his hands off. "So what did –"
"I worked out why Odin brought me here and raised me beside you," Loki said, his grin falling away into something more watchful.
"Oh." Thor was intrigued. He'd given up wondering that himself; Loki was here, Loki had always been here, Loki was as good as his brother. He sat down on the edge of the falls again, on the solid rock this time rather than the iron railing. "Father said it was to do with finding a way to peace with the Frost Giants; some possibility of an alliance."
"And that he would explain more when we were older," Loki agreed, impatient. He lowered himself next to Thor, loosely curling his arms around the lower rail. "At first I thought I must be a hostage for Laufey's good behaviour, but nobody from Jotunheim has ever tried to bargain for me, or shown any interest in my wellbeing, so I don't think my value as a hostage is very high."
"You're a prince of Jotunheim! And my brother!"
Loki rolled his eyes. "An-y-way," he said. "Then I thought my value must be for what Asgard could observe from me, about the natural ways and weaknesses of Frost Giants."
Thor wrinkled his nose.
"Then," Loki said, kicking his legs over the drop, "I thought perhaps I was going to be raised as a spy, loyal to Asgard, who could pass as Jotun."
"Really?" Thor burst out.
Loki scowled. "I know, it was silly," he said. "Who would trust me to stay loyal to Asgard, as a spy?"
"That's not –"
"But I've worked it out," Loki said, turning to face him again. He looked expectant, as though Thor should be able to guess now.
"And it's obvious," Loki said. "Two princes, raised together, and talk of peace and alliances. He means us to marry."
For a moment Thor stared at him. He considered the idea from all sides, cautiously. Loki's mouth acquired a small, sardonic lift. "Yes," he said. "So now –"
Thor whooped. "That's brilliant," he said. He grabbed Loki's shoulders, gripping hard. "If I have to wed, I would wish of all people it could be my brother!"
Loki blinked, his face going very blank as he parsed this. His eyes had an oddly blind look.
"... yes, you would think that," he said after a moment, his voice shaking. Thor wasn't sure if he was laughing or not. Loki was usually laughing at Thor, so it was probably the safe bet.
Thor grinned anyway. "What do you want for your wedding gift? If I start making it now it could be something epic by the time I give it to you."
Loki stepped away, breaking Thor's grip on his shoulders. "I'm not going to do it," he said irritably. "I have years now to work out how to trick our way out of it; that's the point. That was the whole point of telling you, you idiot."
Thor stepped quickly forward, catching the flicker of motion and switching direction at the last moment so that he caught Loki rather than the illusion he had sent in the other direction. They crashed to the rock, Loki cursing inventively at Thor. Thor rolled them so they were face to face, so close he was in danger of going cross-eyed. "I guess," Thor said, grinning like a dog, "I must have years to make you change your mind, then." His grin softened into a smile as Loki stared up at him, honestly startled. Then he gave an appreciative oof as Loki got in a good knee to his stomach.
They rolled again, Loki finding a bit of leverage to get them rolling away from the cliff edge, which Thor judged unadventurous, but was willing to overlook. Thor finally got them wedged under the overhang of another rock, Loki pinned beneath him.
"You're a maniac," Loki panted.
Thor was distracted from answering right away by the sharp motion of Loki's ribs beneath his, always so close to the skin. He moved his hands to Loki's wrists, grinding both their hands into the rock when Loki started to twist in another direction. Loki rolled his eyes, still breathing fast. Thor lowered his head, pressing their foreheads together, their breath mingling. "No, I'm not," Thor said. He felt his breath disturb the hair on Loki's temple. "This is a good idea."
Loki drew in a breath: careful, still. Thor felt it with his whole body.
"Oh," Loki said. It was a very quiet sound.
Then he made some movement with his hand, pinned above his head by Thor's, and a small ball of energy was suddenly rocketing about the small space. As it tore loose the stone they were wedged beneath and threw them both sideways, Thor had time to remember, with a fierce and terrified joy, that Loki always had been the crazier of the two of them.
He windmilled his arms and legs as he fell, letting out a wild ululating yell, and was still doing it when they finally hit the pool below.
He swallowed rather a lot of water.
For the prompt: Thor/Loki, arranged marriage.