As a little kid, my son Henry loved meeting new people, and preschool made him almost dizzy with joy. While we walked back to the car at the end of the first day -- he was doing this little half-bounce, half-skip dance of pleasure -- he kept saying, "Thank you for my new school, Mommy. Thank you for giving me this school."

Henry's natural exuberance was contagious, and everyone loved him. But in third grade, the other boys began to play football every day at recess, a game Henry truly hates, and the whole dynamic started to shift. For a while, he kept playing tag with the girls, even though the boys gave him unending grief about it. But then some of the girls started a private club, and some of the other girls started a rival club, and some of the girls in the original club got mad and started a third, even more exclusive club, and none of the clubs allowed boys.

Henry decided he had to give football a try. But after he got knocked down a few times, he gave up. "I'm sure you're not the only boy who doesn't like football," I said. "What about your friends in chess club? What about the other Cub Scouts?"

"Some of them don't like football, either," he said, "but they play because they don't want to be an outcast." I knew what he was thinking -- "an outcast like me" -- and it broke my heart.

Outside of school, kids will play with whomever they've got to play with -- siblings, children of their parents' friends, next-door neighbors. Any playmate is better than no playmate, so the dynamic is very flexible.

But in school, where there's a wide choice of friends, all bets are off. Children may pair off with a "best friend," join a group that's constantly fighting, feign interests to gain acceptance, even reject other children for no reason. For parents, it's bewildering because it all happens behind closed school doors. That's why Parenting set out to discover what's really going on in that social minefield of shifting alliances and unexpected explosions -- so you can help your child through the inevitable stumbles.