A viral video of two diaper-clad babies babbling in the kitchen has people wondering what the tots are talking about.
Eighteen-month-old fraternal twin boys Sam and Ren appear to be having a grown-up conversation complete with questions, answers, facial expressions and gestures -- even the odd laugh. But they aren't speaking English.
"These kids are right on the cusp of language," said Stephen Camarata, professor of hearing and speech Sciences at Vanderbilt Kennedy Center in Nashville Tenn.
Instead of producing words, the boys are making different sounds in the tone and rhythm of speech.
"They're using the intonation patterns of sentences -- imitating sentences in a crude way," Camarata said. "It's one way that children learn how to talk."
"Even before they have words, they know how conversation works," said Dr. Roberta Golinkoff, education professor and director of the infant language project at the University of Delaware in Newark.
"They're producing syllables emphatically and using them for communication purposes," she said. "They're having a ball."
Eventually, Sam and Ren will start replacing bits of babble with English. But for now, the boys are content with their improvised idioms.
"They're laughing and grinning and imitating," Camarata said. "With twins you've got two kids at exactly the same developmental level going back and forth and having a blast."
Despite Sam and Ren's limited lexicon consisting mostly of "da, da, da," they have a shared understanding of the matter at hand, even though their audience may not, according to Karen Thorpe, a professor at the Queensland University of Technology's School of Psychology and Counseling in Queensland, Australia, who has published several papers on language development in twins.
"I liken shared understanding to what we often see in married couples -- they have been together a lot and therefore some things do not need to be spoken, or limited communication is enough to convey the meaning," Thorpe said.
In rare cases, twins develop and hone their own secret language that only they can understand.
Catherine Brady, mother to identical twins Austin and Landon Grant, who will be 5 years old in August, said she struggled to interpret her sons' private "twin talk."
"They would make up words that they both used, but I was never able to discern a distinct vocabulary," Brady said.
Every night after Brady tucked them in, Austin and Landon would chatter in the dark in a special dialect speckled with references to the children's television show "Blue's Clues."
A 2001 study by Thorpe and colleagues published in the International Journal of Language Communication Disorders found that twins who still used a secret language by age 3 had poorer cognitive and language functioning and highly dependent relationships.
But with help from the Massachusetts General Hospital Institute of Health Professions' Speech-Language Pathology program, Austin and Landon dropped twin talk in favor of English at age 3.
Though rare, twin talk has been the subject of research studies dating back to the late 1800s, and featured in popular TV shows.
Identical twins Katie and Emily Fitch use a secret language to communicate in the British series "Skins." So do Springfield's purple-haired duo Sherri and Terri in "The Simpsons."
Baby Babble: The Cusp of Language
Sam and Ren's bantering babble caught on tape illustrates how in tune the boys are with each other, Thorpe said.
"We are most tuned in to those we spend most time with, and twins are siblings who spend most time alongside each other throughout life," she said.
But it also offers a sneak peak into a fleeting phase of child development.
"There are two developmental achievements being consolidated here; the first is turn-taking and the second is imitating a complex pattern that requires retaining that pattern in the mind to adequately repeat it," said Dr. George Scarlett, assistant professor and deputy chair of the Eliot-Pearson department of child development at Tufts University in Medford, Mass.
Scarlett said turn-taking -- a uniquely human behavior -- is no trivial achievement. Nor is pattern imitation, which is one way young children come to organize their experience. And both are precursors to communicating with words and gestures.
"Babies are wired for communication from the start and we see here a fine example of how sophisticated and beautiful communication in even young children can be," Thorpe said.
Twins are often slower to speak than their singleton counterparts. But Camarata stressed "that's not a bad thing," and said Sam and Ren are "right where they should be."
"I would hope that parents aren't watching so closely that they would not stop to enjoy this moment," Camarata said. "Everybody who sees this should just smile."