At 7pm, in a packed fourth-floor auditorium overlooking a street clogged with people heading home, hundreds of South Korean workers, many still in uniform, take their seats for an after-work class.
Within minutes, they are screaming and guffawing.
"Laughter makes you feel good," the instructor, Joseph Lee, tells the students -- 300 postal workers in the Kwangjin district of Seoul.
"If you feel good, it helps you make your customers feel good. So laugh until your back breaks, until your stomach muscles cramp and until your bellybutton pops out," he said.
In South Korea, where looking cool has often meant looking serious, people are learning the value of a good laugh.
In a country where smiling has traditionally been frowned upon, some people now consider laughter a business skill that is increasingly necessary as customers demand better service.
"Our people have difficulty laughing," said Lee, noting that centuries of Confucianism had taught Koreans to value the solemn more than the funny.
Traditionally, the men considered most attractive were humorless and stern, while women were taught not to laugh during matchmaking sessions or they would risk never giving birth to a boy. Children, meanwhile, were told that laughing too much "drives away good luck."
In modern times, South Koreans have had to come to terms with a grim social mood set by war, decades of dictatorial rule and a headlong rush toward industrialization.
But that mood is changing now, in part because of the popularity of laugh therapy.
In classes offered by local governments and hospitals, instructors preach the healthy effects of hearty laughter.
They cite studies that claim that laughter can stimulate the respiratory system and blood circulation, ease the pain of arthritis and prevent everything from the common cold to cancer.
But there are also business incentives.
The national postal service is among the public corporations that are capitalizing on the value of laughter.
Although the postal service was once a monopoly, it now faces competitors in parcel delivery service.
In the Kwangjin post office, a brochure calls on the office's 500 employees to smile at least once an hour, regardless of any particular reason.
The interest in laughing classes is partly a response to the economic slump, said Han Kwang-il, director of the Korea Laughter Center, who gives 15 lectures a week.
"People want to blow away their economic gloom with gut-busting laughter," he said. "But they don't know how to laugh and where to laugh."
Park Dong-sun, director of a laughter therapy center, Hahaha Korea, said that smiling faces would go a long way toward making South Koreans appear more attractive overseas.
"To people who don't know our manners, South Koreans' rigid look could be taken as an insult," Park said. "The next big jump in our economic growth will come when our people start laughing more."
Many postal workers said they found their company's laughing campaign inspiring.
"In my photo album, I could see that the older I become, the less smiling I was," Ko Ae-ran explained, looking flushed after a session.
"This laughing therapy really is changing my outlook on life. I feel a new energy flowing through my body," she said.
Lee said that as South Koreans aged, laughter declined. Surveys show that an average Korean spends the equivalent of 88 days laughing during a lifetime, and that about 70 to 80 percent of the laughter occurs before age 20.