Looking for the secret to a successful retirement with your partner?
Try watching two children in a sandbox—a boy playing with toy trucks and a girl using a pail and shovel to build sand castles.
Chances are good that the toddlers will be quite content. They'll be next to each other, or moving farther apart; silent at times, and babbling to themselves or each other at other times. They will no doubt resolve minor disagreements if they occur—whereas they would probably be fighting if they tried to play with the same toy in that sandbox.
It's a phenomenon called parallel play—two individuals engrossed in separate but parallel activities. And it might hold the key to happiness in later life.