If you serve up a smile to people, they usually bounce it back. Hit them with a snarl and watch them silently cringe.
Research shows facial expressions and the moods that accompany them are contagious.
You can use the infectious effects of a grin to jump-start an optimistic outlook in yourself by sending others what you want them to lob back at you.
So serve it up like Nadal.
Just for today.
