Deciduous teeth first teeth seen in the mouth are the mandibular centrals and the last are the maxillary second molars. The primary teeth are made up of central incisors, lateral incisors, canines, first molars, and secondary molars; there is one in each quadrant, making a total of four of each tooth. All of these are gradually replaced with a permanent counterpart except for the primary first and second molars; they are replaced by premolars. The replacement of primary teeth begins around age six, when the permanent teeth start to appear in the mouth, resulting in mixed dentition. The erupting permanent teeth cause
I Should Have Known Better solo) and thus draws a line under a significant period of their early music. The song's middle sixteen section features George Harrison's brand new Rickenbacker 360/12 12-string guitar. The mono and stereo versions have slightly different harmonica introductions. In the stereo version, the harmonica drops out briefly. Also, a noticeably clumsy and audible tape edit is heard during the second chorus between "You're gonna say you love me too, oh," and "And when I ask you to be mine." In the UK, "I Should Have Known Better" appeared on "A Hard Day's Night" and was released on 10 July 1964.