Laugh

"Laughter increases white blood cell activity and changes the chemical balance of the blood. This is believed to boost the body's production of chemicals needed for alertness and memory. Laughter reduces stress, and low stress enhances the brains' receptivity to learning. According to researchers, laughing (having fun) also boosts the body's immune system for three days - the day of the fun and the next two." (from Start Smart!: Building Brain Power in the Early Years by Pam Schiller).

Personally, I just love a good belly laugh, don't you? And now I know it does me good!