There are specific sensitive/critical periods in development during which acquisition of certain skills is substantially easier than at other times. In the animal domain there are critical periods that are strictly timed. A good example of such is imprinting in birds; when a duckling is born, the first creature seen, even a human being, will be followed around like the mother duck. In humans, the prime example of a critical period is that of language learning; if a child is not exposed to language during the first few years of life, it will be very difficult (though not impossible (Krashen, 1973)) to acquire language later in life. The critical period for language learning was originally proposed by Eric Lenneberg. He hypothesized that lateralization of the brain (that at the time was thought to be completed after puberty) would be critical determinant for language learning ability (Lenneberg, 1967).