Hahaha! The average four-year-old child laughs 300 times a day. By contrast, it takes more than two months for the average 40-year-old adult to laugh that many times. This week, we talk with behavioral scientist Jennifer Aaker of Stanford University about why so many of us fall off a “humor cliff” as we become adults. Plus, how we can inject more laughter into our lives, even during the most difficult of times.