SOME people are keen learners of many foreign languages: they find it enjoyable to rack up one, then another, then another, reading, practising, brushing up, seeking out any opportunity to use them. They are usually proud of this devotion. (Your columnist must admit to being a member of this odd tribe.) But the longer a language-learner spends on the hobby, and the greater the numbers of languages studied, the harder a simple question becomes. Often asked, it is impossible to give an easy answer: “How many languages do you speak?” The more languages one has studied and the more experience one has, the more the answer feels like “none!” I have learned to give a numerical range and a lot of hemming and hawing.

Ken Hale, a linguist at the Massachusetts Institute of Technology, was renowned among colleagues for picking up languages seemingly instantly. It is said, perhaps apocryphally, that he learned Finnish on a flight to Helsinki. But he insisted that he “spoke” only English, Spanish and Walpiri, an Australian aboriginal language. The rest he merely “talked in”. Tim Doner, an American teenage polyglot (see The Economist’s multilingual video interview with him here) is much the same. A video about Mr Doner has the title “Teen speaks over 20 languages”, but Mr Doner laughingly says only that he is “very comfortable” in four or five.

Anyone who has gone far in even a single foreign language knows that competence is graded: there is no magical day when someone pins a gold star on your lapel and says: “Congratulations.” It is more like driving into town and listening to a radio station that at first comes in weakly, with heavy static, and only gradually becomes clear. At what point did you start getting that station?

Lacking a clear line, language-learners have all manner of ad-hoc ways of describing when they finally got it. Some will say that the day they began dreaming in a language meant that they had it. But this is hazy; dreams, played in one’s own head, say little about real-world competence. Others recall the first conversation in which they did not have to struggle; this, at least, is a better rule of thumb. Others set the bar higher: only when you understand jokes do you really know the language. This sets the bar perhaps even too high. You can communicate all manner of things without having the fingertip-feel needed for wordplay.

Competence also has domains. The simplest are the staples of getting to know someone, taught in every language class. What is your name? Where are you from? Why did you learn Russian? You speak such good Portuguese! People from your country don’t learn Polish very much. Is this your first time in China? And so on. Most polyglots will have repeated these opening conversational turns a hundred times, and so the initial meeting with the delighted foreigner is bound to impress.

But once the conversation goes into unexpected territory, things can go badly quickly. The reason is the huge vocabulary needed to talk about all of life’s many domains. It has been estimated that the average English-speaking adult knows the meaning of about 30,000 words, far more than most people think. A fluent speaker of a foreign language might know just a tenth of that number. This means that, a good accent, rhythm and grammar notwithstanding, the intermediate-to-advanced learner is likely to flail discussing the value-added tax, running style or camera lenses.

Your columnist knows from experience: I have flailed on all of the above in languages I would otherwise say I had spoken well for years. When I heard “90 dólares más IVA” in Spanish for the first time, I thought was “90 dollars masiva”, or massive, wondering what this curious expression meant. But it just meant “plus VAT”. When a French colleague asked about running in a place called “Pienu”, I had no idea where that was, until he switched to English: “barefoot”, or pieds nus. And “focal length” is Brennweite in German, which no one would easily guess, as it looks like it means “burn-width”. (It does make sense, though: focused light rays can burn, which is why “focus” comes from the Latin for “hearth”.)

Repeat these disappointing experiences a couple of hundred times in a life, and one becomes very cautious about casually rattling off a big number of languages spoken. Your next conversation may be a breeze, or a frustration. Perhaps it is best to imitate Mr Hale and talk about “talking in” foreign languages, or to take after Mr Doner, and merely say which ones you are comfortable in. Ziad Fazah, the man whom the "Guinness Book of World Records"said spoke 58 languages, was humiliated in a Chilean television performance. Peppered with random questions from the audience (“What is the only man-made structure visible from the moon?” in Chinese, “What day of the week is it today?” in Russian) he flailed repeatedly, in a YouTube clip that serves as a well-known cautionary tale among polyglots. “The bigger they are, the harder they fall,” applies to more than physical size. Braggarts beware: real-life language learning is a constant exercise in humility.