I remember watching a father play with his little boy, repeatedly throwing him in the air and catching him just before he hit the ground. The child is relaxed and having a great time saying, "Do it again! Do it again!"

I thought, If that was me, I 'd be stiff as a board .

"Can you explain why he's so relaxed, even when he's out of control?" I asked the father.

"It's very simple," he said. "We have a history together. We've played this game before, and I've never dropped him."