TIMOTHY DONER looks like an ordinary American teenager. Medium-height and slight, he arrives in a grey T-shirt and jeans. As he is being miked for his interview, our producer asks him a standard question to get him talking, so that she can check his voice levels: "What did you have for lunch?" He hasn't yet eaten today, having only just got out of bed. It is a little after two.

He's entitled to a bit of sleep; he obviously puts in long hours. The breathless title of a YouTube video about him rocketing around the internet tells why: "Teen Speaks 20 languages." When I ask him how many languages he speaks, he offers a different answer, though. One, really. That is, English. He is "very comfortable" in four or five others, and "very serious" in studying two to four more at any time. Others he "dabbles in".

He could be quite a bit more boastful than he is. The videos going round the internet show him chatting with a bookshop owner in Urdu, responding to a teacher in his Mandarin class, discussing the similarities between Hebrew and Arabic in Arabic, and giving short monologues in languages from Indonesian to Swahili to Ojibwe (a North American Indian language). The speed with which he learns, the comfort with which he speaks, and of course his youth make this nothing short of astonishing.

So we invited Mr Doner to our studio in New York to talk with several native speakers in an unscripted conversation. First in French, then in Russian and finally in Mandarin, he overcame some initial nervousness to chat about French street slang ("verlan"), his family roots in eastern Europe and Chinese food. My colleagues were told to try to ask him a question he might not expect; he handled each of them with ease, never once asking them to repeat themselves. Native speakers will see tiny mistakes—a missed Russian case ending here or a Mandarin tone there—but they stick out in a stream of otherwise relaxed and easy conversation. And it should be stressed that we did not test his best foreign languages, which are Arabic and Farsi. He has had formal instruction only in Arabic, Farsi, French, Chinese and a bit of Japanese, Hebrew and Swahili. Others (like Russian, a hard language he speaks comfortably) are self-taught. He seeks out opportunities to practice less common languages, like Ojibwe and Wolof, on Skype, and finds many a New York cab driver to speak Hausa.

Many people want to know what makes a voracious language-learner tick, a question Michael Erard set out to answer in his book "Babel No More", reviewed here. Most hyperpolyglots are male. Many have some combination of being gay, left-handed or ambidextrous, and poor in visual-spatial skills. A surprisingly large number are boorish anti-social types. A variant of the theory has it that hyperpolyglots might have a highly "male" brain, driven to systemising rather than empathising. (A subset of this theory is that autism is the result of an "extreme male brain".)

Mr Doner hardly fits the profile (except for being a left-handed male). He has the will to sit and memorise verb tables, as one must do to come as far as he has. But he is a sociable and confident teen with a ready smile. He loves memorising pop lyrics and watching movies. He virtually inhabits the languages he speaks; as a colleague said on seeing his video, "he shrugs like a Frenchman and frowns like a Russian." Most of all, it is obvious how much he enjoys speaking his languages with other people, not just learning them for the purpose of translation or reading (or boasting).

What else is he good at? He gets good grades in maths, but finds it frustrating, and struggles with physics and chemistry. He loves history, a big motivator in his language-learning. His father was once a professional pianist, and the young Mr Doner says that after a few years of lessons, he could "sight-read and accurately play pieces in one go", though he is out of practice now. He can also quickly learn things by ear. This is perhaps the most intriguing clue to his ability—not just a "systemising" brain, but one highly adept at processing and producing in a given compositional system (musical or linguistic) on the fly, plus a world-beating auditory ability.

What's next for a 17-year-old hyperpolyglot? He still has a year of high school, and then university, where he plans to study linguistics. He has already taken an interest in language science alongside all of the languages themselves. In an e-mail to me, he recommended Mark Baker's "The Atoms of Language", a fairly difficult work of Chomskyan theory (though written for lay readers). In our video, he mentions skipping over easy languages like Spanish, instead choosing new languages like Ojibwe, because they pose novel challenges like agglutination or ergativity.

And after college? Everyone, naturally, asks him if he will be a spy, which he laughs off. In any case, he is by now too well-known to disappear into the shadows. Diplomacy interests him, though. And America's foreign service would be lucky to have him.