"You're a poop-head," says Sara her eyes glinting with anger. "Am not, booger-brain," replies her friend furiously. The two stand nose to nose, glaring at each other hatefully. In an hour, though, they'll be playing happily -- "best friends for life". Several kids, aged ten, are playing a card game. "Gotcha! Ha, ha, you're a loser!" gloats one, waving his cards around. Two others have their heads together, whispering conspiratorially, "Let's gang up and get him," they say, speaking about the "loser." Meanwhile the "gloater" is rolling around on the floor gleefully...