CAIN BIT MY FINGER:

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.

In Britain, some commentators have struggled to fathom the appeal of the video and sequels showing the boys and their brothers, Jasper, 3 ½, and Rupert, 8 months, engaging in nonbiting activities. "They are not, to put it politely, exceptional," wrote Terence Blacker in The Independent.

That is not how the world sees it, and Mr. Davies-Carr said he had thought long and hard about how to "monetize" in a responsible way what has become a very valuable property.

He is a YouTube "partner," meaning that the family shares in advertising revenue generated by the site. The boys make the odd commercial. A new "Charlie" app is in the works for iPhone and Android devices. There is talk of children's books and a YouTube channel.

The family is represented by a company called Viral Spiral, whose clients include the people behind the sneezing baby panda video and the babbling twin baby video. Damian Collier, who founded the company about a year ago -- the Davies-Carrs were his first clients -- said he had been impressed by their approach. "When I first met up with Howard, it was clear that he was enormously concerned to protect his boys and make sure anything done was tasteful and not exploitative," Mr. Collier said.

It has been a hard line to walk. 

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