Sam always liked math, from the moment Dean sat him down in a kitchen chair and started teaching him to count, dropping silver bullets into his open palm like metal raindrops. One, two, three, four.
Math makes sense when a lot of other things in his life don't. Six popsicles means one for him and one for Dean every night for three nights, whether or not his Dad comes home or Dean carries a knife around for no reason.
Math is simple when there are so many things in his life that are hard to explain. Four hundred twenty three dollars and seventeen cents means rent for the month and a meal for the night, even if Sam can't tell his teacher why he still hasn't bought the books required for class.
Math is consistent when Sam learns there's little in his life that stays the same. The derivative of the natural log of x is always one over x, no matter what town they're living in or which monster their Dad drags them after that week.
Math is safe when Sam sometimes forgets what that word even means. He can do thirty stitches in six and a half minutes, thirteen seconds per stitch, give or take some time for Dean cursing and raging or knocking Sam's hands aside as Dean lurches for the whiskey bottle.
When it comes to math, Sam knows the answers. He knows what to do when he's faced with finding the limit of the sine of x divided by x as x approaches zero. And if he gets it wrong, no one gets hurt.
Dean used to comment when Sam would lay awake in the back of the Impala and count streets signs, other cars, anything he could see outside the world of the car. Sometimes Dean would join him, and it'd become a game until their Dad told them to be quiet and go to sleep.
Dean didn't say anything when Sam started whispering multiplication tables into the darkness of shared motel rooms in order to fall asleep. He got used to ignoring the mumbled rules of Euclidian geometry in the early morning or the staccato breaths of the numbers of Pascal's triangle while they load their guns before a hunt.
Dean doesn't know Sam's first conversation with Jessica was about inverse trigonometric functions. He doesn't understand the way Sam sometimes scribbles equations on diner napkins, solving them over and over to prove to himself he still can. He doesn't hear Sam put his fist through a window the first time he can't remember how to do Poisson distribution.
But sometimes, when things get especially rough, when Sam can't sleep at all, when the only math he is doing is dividing the number of their scars by the number of days without Dad or Jess and trying to arrive at a conclusion Is it worth it is it worth it, Dean stops the car. They climb out, in the middle of a field by some anonymous highway, tire tracks cut through the grass like knife blades in the dark. Dean grabs them both a beer and they sit on the hood and stare at the stars. They don't speak, but Sam's counting in his head, one two three four. It's logical, it's simple, it's consistent. It's safe.
Dean never really liked math, because he's not a gigantic dork like Sammy, but he doesn't mind it so much when that gigantic dork sits next to him with the night sky reflected in his eyes and a smile on his face.