The pebble was tossed when a middle-school student in upstate New York posted a 10-minute video on his Facebook page.
The video, showing four other seventh-grade boys cruelly taunting 68-year-old bus monitor Karen Klein, was quickly uploaded to YouTube.
And the ripples began.
Millions of viewers from around the world watched her humiliation. There were cries of indignation and sympathy, retribution and recompense. Through posts on social media and the user-generated news site Reddit.com, word spread geometrically, leading to a fund drive that began with a modest goal of $5,000 to help Klein take a nice vacation and scrub the foul memories of the last days of school from her mind.
By Friday afternoon, the drive had sailed past $550,000, with donations from more than 25,000 people.
Even in an increasingly connected, fast-moving world of information flow and echo, the response to Klein's plight is a stunning example of the power of people in the new, Me-Media era.
"Oh, my God," Klein told The Associated Press in an interview Thursday, when the total was around $370,000. She said it was "weird, very weird" to suddenly be an international celebrity and joked she'd have to go out in public disguised by a wig and dark glasses.
"I appreciate everything so much," she said. "It's just hard to believe strangers, people I never talked to, never seen, will send me a message saying, 'We love you, we think you're a great person.'"
Lee Rainie, director of the Pew Internet Project and author of "Networked: The New Social Operating System," said these kinds of moments have occurred before, but he still called the sheer volume of the response to the Klein video "head-scratching."
"It kind of feels like there aren't boundaries to this stuff," said Rainie.
The formula is pretty simple, Rainie said: A lot of people passionate about what they do keep vigilant eyes on the Web and react instantly when something offends or delights. The speed and reach of the Web do the rest.
"It's clear there are any number of watchdogs, you can call them; cultural or civic observers who scan YouTube or pictures for evidence of bad behavior," he said.
"Once it sort of gets in the line of vision of the people who get mad about these things, they use the Web to sort of, first, exact their version of justice and secondly, to help people who are clear targets or victims. Obviously, there's a cascading networking effect on this."
The verbal abuse was captured in a 10-minute cellphone video recorded Monday by a student of Athena Middle School in the Rochester suburb of Greece. The video shows Klein trying her best to ignore the stream of profanity, insults and outright threats.
One student taunted: "You don't have a family because they all killed themselves because they don't want to be near you." Klein's oldest son killed himself 10 years ago.
Eventually, she appears to break down in tears.
Max Sidorov, the 25-year-old Canadian man who started the fund drive on the site Indiegogo.com started with modest goals. In an interview Friday with The Associated Press, the kinesiologist and nutritionist said he was bullied when he came to Canada from Ukraine, so he empathized with Klein and likened the taunting to "attacking the little guy."
"Only in my wildest dreams did I think the amount of money raised would reach this much. It's very amazing to put it lightly, the amount that she's got," he said.
Not all the feedback has been positive. Police in the Rochester suburb of Greece, N.Y., stepped up patrols around the houses of the middle-schoolers accused of taunting her. Police didn't name the boys, but their purported identities leaked out on the Web and at least one received death threats.
"There's a danger it turns into a vigilante sort of mob and people are misidentified," Rainie said. "These things can move very rapidly out of hand and make things miserable for the wrong people."
Police said Klein does not want the boys to face criminal charges, partly because of the storm of criticism leveled at them. The district will pursue disciplinary actions against all four students.
In the AP interview, Klein asked people to leave the boys alone.
"Threatening them? No. That's not the way to go about things," she said. "They're just kids."
"I don't want to judge anybody or put them in jail or anything like that. I just want them to learn a lesson."
Stevens reported from Albany, N.Y.