It is a memory that gives him the idea.
When they were children, Thor and Loki would sneak into Idunn's orchards and climb the branches of the tallest apple tree. There was a game they would play as they climbed, each searching for the juiciest, plumpest apple. Once each had found their pick, they would sit on a branch, pressed close to each other and compare. And then Loki would take the best apple, cut in half, and hand one half to Thor. They would sit there in their tree, laughing and each eating their half, juice dripping down their chins.
Thor will never forget how his brother's green eyes would glint in the sunlight with simple joy. Before he learned to hide himself away, before his every expression became the result of careful calculation.
It was there that they shared their first kiss, and Thor still remembers the warm heat of the sun on his face, the sweet taste of apples on his Loki's lips. As they got older the tradition continued, although it became less and less frequent.
Now he only ever sees Loki staring at him in anger from the opposite line of the battlefield.
Asgardians are not immortal, not exactly. Their lives extend for so long only through the magic of the apples that grow in Idunn's orchards. Loki is a wanted man in Asgard and he cannot even show his face there, let alone ask the goddess for an apple (which she would no doubt deny him anyway). Thor doesn't know how long it has been since Loki had an apple. The very thought of his brother aging and dying like a mortal is something Thor cannot bear.
And so Thor makes a decision.
There are not many he can trust to help him, and so he goes to his mother and tells her of his intentions. Frigga's knowledge of magic is limited, but she tells Thor enough to attempt a simple summoning spell and magical pendant that will hide him from Heimdall's gaze. The spell is not powerful enough to force Loki to him, but he knows that being backed into a corner is not something his brother appreciates anyway. It is more like… a call placed by one of the mobile devices the Midgardians favor. A call that Loki can decide whether or not to answer.
He goes away to a secluded forest to perform the spell. Although subterfuge was always more of Loki's area, Thor does not want the other Avengers to know of his actions. They do not understand his relationship with Loki, they never have.
He loves Loki like no other. Even when his body ceases to draw breath and his soul goes to Valhalla, he will still feel the same. He can no more stop loving Loki than he can stop fighting for what is right, even though to many people those things appear to be at odds with each other.
Once the spell is cast, he sits down on a hollow log and waits. He cannot be sure that Loki will come, but he knows his brother well. He hopes his summons will pique his brother's deep-rooted sense of curiosity.
Thor feels a flutter of wind and he looks up to see Loki standing before him. He wears his armor but not his helmet. His cheeks are sunken in and his eyes smolder with contempt. And yet he looks so beautiful that it makes Thor's heart clench.
Thor is the first to speak.
"I was not certain you would come."
Loki gives a short bark of a laugh. "The Mighty Thor attempting to use a summoning spell? Such a great warrior, stooping to the arts of us cowardly sorcerers? I have to admit I was most intrigued."
There are many things that Thor wants to say. That he does not think that sorcery is cowardly and that he's sorry for the times he belittled Loki's talents. But these are things he has told Loki before and he knows they will fall on deaf ears. And besides, he has called Loki here for a very specific purpose.
"I wish to make a bargain with you, brother," Thor says.
Loki laughs mirthlessly. "Do you truly think it wise to make bargains with me, Thor?"
"Perhaps not, but I have never claimed to be wise."
"So speak, then. Tell me about this foolish 'bargain' you wish to strike with me," Loki says as he crosses his arms across his chest.
Thor picks up the small bag at his feet. He stands and crosses the distance between them until he stands before Loki, but not too close. He doesn't want Loki to run, not before Thor has spoken his piece.
Thor reaches into the bag and pulls out the apple and a small knife. He cuts it in half and lets the bag and the knife fall to the forest floor. He holds the halves of the apple, one in each hand. One hand is held closer to himself and the other extended towards Loki.
Thor sees the moment the realization crosses Loki's face. It is fleeting, but his expression turns soft. In that moment he almost looks like the brother Thor once knew. But as soon as it appears, it is gone again and replaced with a scowl.
"You are a sentimental fool," Loki spits out.
"And yet you remember too," Thor counters. "Perhaps I am not the only sentimental one among us, brother."
This only makes Loki's scowl deeper. He turns his face away from Thor and looks out into the distance. Thor imagines that he can hear the wheels in Loki's mind turning as he processes this new information.
Loki looks back at Thor, his expression wary.
"And what would you have in return?" he asks.
Loki raises an eyebrow at this.
"Each month I will call for you and bring an apple from Idunn's orchards. For each apple I share with you, I ask for one day," Thor continues, "one day where all grievances against each other are forgotten and we pass the time together as brothers, as we once were."
"You are an even bigger fool than I imagined," Loki says, voice laced with contempt. "Too much has passed between us for it to be forgotten, even for a day. What you ask for is impossible!"
Thor laughs. "What does 'impossible' matter to you, brother? You who have only ever seen the word as a challenge to overcome."
"I could say the same of you," Loki retorts.
"Aye," Thor nods. "So then, do we have a deal?"
Loki steeples his fingers and places them against his lips as he considers things for a long moment.
"Yes," he says finally.
They spend that day in the forest, wandering down the trails together in a way reminiscent of the quests of their youth. Thor catches a fish and they cook it over a fire that Loki conjures. Thor tells Loki about the wonders he has seen on Midgard, all of the places he wishes to show Loki.
Loki is stiff and distant, but he is not actively hostile. When the day is over and he takes his half of the apple, he nods at Thor in acknowledgement before he disappears in a puff of green fire. Thor considers it a victory. A small one, but a victory nonetheless.