But first, Post had them fill out a questionnaire that asked how they felt about life, like how often during the week they felt calm and peaceful.
Children for Children, whose mission is to get children involved in giving, agreed to help us, as did the Salvation Army, which has many different programs, from soup kitchens to after-school activities for kids.
Then off they went -- bringing donated books to children at an elementary school, then reading to the kids and making scarves with the kids. One spent time in a truck handing out food to the poor. All four worked at a Harlem soup kitchen.
One week later we had them answer that questionnaire again.
This time their answers about how often they felt "calm and peaceful" changed from some of the time to most of the time.
Post said, "In fact, just seven days of activity was a kind of a transformation."
One of our volunteers, Diana Sanchez was surprised at how strongly the experiences affected her.
"They were so grateful for me doing that, but it was just peas and carrots," she said.
When asked whether it had made her feel good, she said, "It did. It did."
Sanchez also spent time with kids at a Salvation Army after-school program.
"Just knowing that after work I was heading over to work with the children, just seeing them smiling -- that just made me feel so great," she said.
Researchers call that "the helper's high."
"The helper's high has been measured physically," Post said. "We know there's an actual physiological state. It's quite euphoric."
The helper's high shows up in MRI brain scans.
People who give money show brain activity that's associated with feel-good chemicals like dopamine -- the same brain activity that happens when you receive money.
National Institutes of Health neuroscientist Jordan Grafman showed us the brain scans.
"Those brain structures that are activated when you get a reward are the same ones that are activated when you give. In fact, they're activated more," he told us.
We asked our volunteers after their week of service who had gotten more out of the experience: the people they helped, or they themselves?
Volunteer Daniel Smith didn't hesitate with his answer. "No brainer. Me, definitely."
Lelani Clark also felt renewed from her single week of volunteering.
"I just felt energized," she said. "We were so caught up in this energy of helping that it was like a buzz -- like a spiritual buzz."
Winfrey's audience members reported that, too. After a week of giving money away, many said they were changed.
Maybe we should call it selfish to help others, because it seems to help the givers more.
"If you want to define selfishness so widely as to include the warm glow that people feel in the aftermath of selflessly giving to others, guess what, we need more of it, not less of it," Post said.
So try it.
Get out and give your money or your time. You'll help someone else. … And you'll feel good, too.
This story originally aired on December 1, 2006.