When you smile, don’t hold back.  Show as many teeth as you can.  See if the observer can count at least half your teeth when you beam at them.  A smile is infectious.  But rather than a nasty virus, you’re spreading mirth and merriment.*  And if you can smile even in the worst of circumstances, then you’ve truly mastered the secret to a happier life.

*Even if you have poor dental hygiene, people will still return your smile, though they may back up a step or two.