The first time Sam could remember Dean with a sunburn was the summer before he started kindergarten, so Dean must have been nine. He'd seemed huge to Sam, doing cannonballs into the pool where they were staying. Sam thought it might have been North Carolina or West Virginia--something with a direction in it--but either way it was hot, and they were bored, holed up at the motel while Dad slept his way through a summer flu.
Sam remembered holding his finger up to the sign on the fence around the pool, sounding out the words printed there. "ALL CHILDREN MUST BE ACCOMPANIED BY AN ADULT." Accompanied had given Sam some trouble; he knew what the rest of the words meant, but even once he stumbled his way through sounding that one out he couldn't understand it.
Dean had rolled his eyes and tugged Sam through the gate. "It means little kids need to be with somebody older, and you're with me."
That had made sense to Sam at the time, and nobody bothered them. There had been a woman sunbathing on one of the reclining chairs, and if anybody had taken the time to worry about the two boys playing in the pool they probably would have assumed she was their mother. In any case, they were both good swimmers, used to playing in the deep bend of the tree-shaded creek that ran behind Pastor Jim's house. Sam couldn't remember a time when he didn't know to float and move himself around in the water.
They had the motel pool to themselves, and they took advantage of it--racing each other in laps, dunking each other, making humongous tidal waves with their arms. They'd been at it for a few hours, and Sam was starting to get hungry when Dad came out, rubbing at the scruffy stubble on his face and blinking in the bright sunlight. Dad shepherded them inside, and tossed a towel over each of their heads, but Sam didn't understand why Dean winced, wriggling his back to get away from the rough terrycloth.
"Dean?" Dad looked confused too, and he led Dean over to the window to get a better look at him in the light. "Ah, Jesus Dean. Why didn't you put on sunscreen?"
Dean shrugged, then winced again. "Dunno. Guess I forgot."
Later on, Sam would figure out that Dean hated being the only one of them who had to worry about putting on SPF 30 before going out in the sun, hated that a few hours in the sun would turn Sam or Dad nut-brown, maybe leaving a hint of pink on Sam's face, while Dean would burn and burn to earn every trace of tan.
That afternoon, Dad gave Dean some Tylenol and went out to the store to get a bottle of clear green sunburn gel that made Dean wince and bite down on his lip. At night, he said it hurt to lay down on the sheets, but Dad put some more of that gel on him and gave him a Benadryl and some more Tylenol, and eventually Dean fell asleep spread-eagle over way more than his half of the bed. He was still asleep when Sam woke up the next morning, and the skin on his back was bright red. Sam reached out a tentative finger and found out Dean's back was warm, too, but then he discovered that if he pressed down that patch of skin would turn white, and then he could watch it fade back from white to pink to red. He amused himself with that for a while, drawing lines and circles and dots on Dean's back until Dad chased him away and gave him cereal and juice to eat at the table.
Dean wasn't allowed to go out in the sun for almost a week, and Sam stayed in with him even though it was boring. Eventually, Dean's back peeled, flaky, paper-like strips of dead skin that were gross and awesome. After that, Dean got better at remembering to use sunscreen, usually wearing a t-shirt at least, and Dad got better at reminding him. Sam started reminding him too, though Dean usually told him to shut it.
Sam didn't get a sunburn of his own until he went to Mexico with Jess over Spring break their junior year. He'd never really bothered with sunscreen, but when he fell asleep on the beach he woke up with the skin on his back feeling tight and hot, and his soap had burned in the shower. Even then, he hadn't peeled, hadn't been anywhere near as red and painful as Dean's sunburn must have been. Jess rubbed aloe on his back and made him use her sunscreen for the rest of the trip, and that hadn't been much of a hardship, letting her use her talented fingers to rub the thick lotion all over his back and chest. The red faded to dark brown within a couple of days, and Sam figured he had one thing he could thank his father for.