When children first begin to talk, they learn perhaps a word or two a week. Then suddenly, at around 14 to 18 months, they seem to soak up new words like a sponge, learning as many as ten a day.

Child psychologists have long been puzzled by this dramatic acceleration, and they have developed a variety of theories to explain it. Some researchers have argued that as the brain develops, it reorganizes to become much more efficient at storing new words. Others have argued that babies are able to use their knowledge of a few words to deduce the meanings of other words.

Bob McMurray, a child psychologist at the University of Iowa in Iowa City, now says that a bit of mathematics is enough to explain the phenomenon. Children's word absorption, he says, is the logical consequence of two basic principles: (1.) children work on learning many words at the same time, and (2.) some words are more difficult to learn than others.