A smile

The power of smile cannot be discounted.  When you smile, it will not only brighten up someone's day but it also keeps wrinkles away.  Most often than not, when you smile at someone, you will get the smile back.  In some cases, however, when you won't get the smile back, don't get discouraged because you will still reap the benefits anyway.  Just keep stretching that muscle and liven  your day.