It's called the "word spurt," that magical time when a toddler's vocabulary explodes, seemingly overnight.
New research offers a decidedly unmagical explanation: Babies start really jabbering after they've mastered enough easy words to tackle harder ones. It's essentially a snowball effect.
That explanation, published in today's edition of the journal Science, is far simpler than scientists' assumptions that some special brain mechanisms must click to trigger the word boom.
Instead, University of Iowa psychology professor Bob McMurray contends that what astonishes parents is actually a fairly guaranteed outcome of the under-the-radar work by tots as they start their journey to learn 60,000 words by adulthood.
If McMurray is right, it could have implications for parents bombarded with gimmicks that claim to boost language.
He thinks simply talking and reading to a child a lot is the key.
"Children are soaking up everything," he said. "You might use 'serendipity' to a child. It will take that child maybe hundreds of exposures, or thousands, to learn what 'serendipity' means. So why not start early?"
Sometime before the first birthday comes that first word, perhaps "mama." A month or so later comes "da-da." It may seem like it took the baby almost a year to learn the first word and a month to learn the second. Not so. He had been working on both the whole time.
Up to age 14 months, on average - and how soon kids speak is hugely variable - words pop out here and there. Then comes an acceleration, and after they can say 50 or so words there's often a language explosion, McMurray says.
What sparks the spurt? There are numerous theories centering around the idea that a toddler brain must first develop specialized learning tools, such as the ability to recognize that objects have names.
The new research doesn't negate those theories, but it suggests "we might be missing the big picture," says McMurray, who developed a computer model to simulate the speed at which 10,000 words could be learned.
He found that as long as toddlers are working to decipher many words at once, and they're being exposed to more difficult words, the spurt is guaranteed.
Consider: Scientists know children learn through the process of elimination. If Mom says, "Please pass me the plate," and the child sees a fork, a spoon and some round thing, by age 2 most will match the new word to the unknown object.
That fits with McMurray's model. As you acquire many words, the process of elimination for new ones becomes easier and vocabulary accelerates.