Researchers had adults, four-year-olds, and two-year-olds say "bed" repeatedly. But scientists filtered the sound so that the subjects heard themselves through headphones pronouncing it as "bad." Adults spontaneously compensated, and changed so that the word sounded correct to their own ears. They wound up saying "bid." Four-year olds also adjusted their speech. But the two-year olds kept saying "bed" even though they kept hearing "bad."