Within the first few days after birth, infants are sensitive to the stress patterns or rhythms of the words they hear. For example, infants can distinguish between a two-syllable word that begins with a stressed sound (like "FA-ther") and a word that ends with a stressed sound ("to-DAY"). Researchers have found that babies pay attention to the stress patterns in words even if the sounds in the words are the same. In one experiment, infants only 3 days old heard the word "Mama" pronounced with a stress on the first syllable (MA-ma) and on the second syllable (ma-MA). Based on how often babies sucked on a pacifier, researchers found that infants can distinguish between the sounds of the words based on the stress patterns. Many researchers believe that this ability helps babies learn to identify word boundaries, which aids in learning language.