The family gets recognized in random places — on the subway in London; in the house, by the washing-machine repairman. Last month, “Charlie Bit My Finger” was mentioned on “30 Rock.” Web-savvy children around the world apparently quote from the video when they get hurt, or want to express mock indignation. There are fan sites, Facebook pages and endless parody videos. A lady in New York volunteered to be the boys’ honorary aunt. Their dentist recently revealed that an acquaintance had named his son Charlie, as an homage.

The video has also changed their life materially. Mr. Davies-Carr will not say exactly how much money they have made, only that it is “in excess of £100,000,” or more than $158,000, and enough to pay for the boys’ education.

Why this particular video?

“Videos are videos. They’re either popular or they’re not,” said Mr. Davies-Carr, who originally uploaded the video as a way to keep a family friend in Colorado apprised of his sons’ progress. The viral part came later, unexpectedly, he said, when the video was apparently picked up by a college humor site and somehow things took off.

If its charm seems straightforward — Harry’s cute accent, Charlie’s maniacal laugh — its unprecedented global popularity is mysterious. We are not talking about “Citizen Kane” here, or even “Porky’s Revenge.” Although videos of cute animals, cute kids and hapless people unexpectedly falling down do better than videos of things that are not cute or funny, figuring out what makes one video touch a viral nerve and another fizzle into unwatched obscurity is one of the great challenges of the YouTube age.