A millionaire with cash to burn is paying it forward in a big way: by hiding envelopes stuffed with $100 across the Bay Area for strangers to find.
The anonymous man told ABC News he's a successful real estate investor who just wanted to give back. READ: Family Leaving 00 Tips for Restaurant Servers
READ: Anonymous 'TipsforJesus' Leaving Thousands in Tips for Servers
"People complain that the price of real estate here is pretty high and I have benefited from that," he said by phone, after ABC News reached him via email. "I wanted to do something fun for the city of San Francisco, something that would get people excited."
The mystery man, who posts clues about his cash drops on Twitter under the handle @HiddenCash, says he plans to expand to New York and Los Angeles soon.
He says he has already given away nearly $4,000 since Thursday night. He's so intent on remaining anonymous that he would only say he's between 35 and 45 years old.
Gleeful followers post photos on social media of the envelopes they find, sometimes taped to ATMs, parking meters or under benches.
Richard Rodriguez and his partner raced to a street in San Francisco's Mission district Sunday seconds after the Twitter account gave a hint the cash would be hidden on a parking meter there, he told ABC News.
"It's about 10 blocks from us, so we did a power walk down there," he said. "When we got to the parking meter it wasn't there and we thought, 'Wow, how could anyone have gotten here so fast? Then a guy was driving by and threw an envelope out the window. I guess he didn't even have a chance to drop it off!"
Inside the envelope was $66 in cash and a lottery ticket Rodriguez cashed in for $5.
"The money was great but it was also so much fun doing something like this. It was a scavenger hunt," he said.
"It's not a lot of money. It's not going to change your life. But the camaraderie it brings out in people is a lot of fun."
Another San Francisco resident, Sergio Loza, said he found an envelope taped to a parking meter after following the clues.
"He said, 'You're usually feeding them but today they're feeding you.' It's like a riddle," Loza said.
The anonymous donor says he'll keep hiding the cash -- usually $100 a pop -- "indefinitely."
"I'm giving right now about a thousand a day," he said. "For me, that's definitely manageable. I know that for most people it's more than they earn, but I'm in the top 1 percent. I can keep doing this."
He hopes more people join in as he expands to Los Angeles this weekend.
At the very least, perhaps it will just encourage more random acts of kindness, he said.
"One example is you go to the toll booth and pay for the five cars behind you. Well, let's say the person behind you has more money and makes more money than you," he said.
"You're still doing a good thing because maybe they're having a bad day or a bad week and you've lifted their spirits and brought a smile to their face. People that don't need money, per se, can still benefit."
Why This Mystery Millionaire Is Hiding Cash Around San Francisco