The Smile

Grinning and burping, giggling and laughing, the twins grew up nursing an inside joke they'd shared since utero.
As infants, their mother would watch them for hours as they lay in their crib laughing at each other. There they'd be, in diapers, face to face, too young to turn over, just bubbling over with hilarity. They would drift (chortling) off to sleep only to awake with loud, perfectly synchronous guffaws, which would set them both off again for another round.
When they were toddlers, learning to walk took them longer than most other kids because every time one would get to his feet, he would catch his brother's eye and collapse, laughing, to the floor. Talking was easier for them, they made it a game of 'who can figure out what we have to say first,' the only trick was getting it out between bursts of merriment.
Upon entering school, they found out that random peals, shrieks and howls were not going to get them on the teacher's good side. Not like they cared, they were having a grand time, but soon their parents wearied of being called into the offices of the principal over and over and finally threatened to place the boys in separate classes. It was then that the boys put their inseparable heads together and developed the secret smile. This smile would express all the mirth and joy of their joke without the laugh riot that ended in parental involvement. They begged their parents for one more chance and the grown-ups (thinking all along that it was ridiculous for anyone to get in trouble for being happy) acquiesced.
The next day at school, the smile came into use immediately, before the boys had even walked in the building they had utilized its power more than once. From their separated desks across the classroom, they shared their joke and no one was the wiser. The day went on; the teacher was astounded. Sure, there were some slip-ups. Sometimes, the joke was just too funny, the smile just couldn't cut it and the hilarity would slip through the cracks. But these times were forgiven for the improvement was still vast. Soon, the two were the best in their class and were even allowed seats together again.
Throughout elementary and middle school, the boys were always together; in the same classes and extracurriculars and sharing the same friends (only ones with good senses of humor, although no one really got the joke), but as high school rolled around, the twins' schedules shifted and differed. They found themselves with incongruent interests and talents and diverging social groups. But they always had their joke and their smile.
After graduation, they attended separate universities in far away places to study totally different things. They both became highly successful in their respective fields. They grew up and got married and had kids.
One day, one was reading the funnies and came across a particularly amusing one. He laughed out loud.
At the same time, somewhere else in the world, his twin smiled a secret smile.