By his third birthday, the child can usually join at least three words and many children are using sentences of four or five words or even longer. Some children may speak very clearly, yet have a poor vocabulary or very immature use of grammar. During the phase of very rapid language and vocabulary acquisition, many children copy whatever is said to them, as if mentally replaying a word or phrase out loud, which helps them to understand better. Primary school-age children are less egocentric and more logical in their approach and are particularly adept at classification and categorisation. In the third year of life, the child acquires various other skills, such as building models or structures with bricks or Lego, using a pencil, doing simple puzzles, playing games with rules, etc. With increasing emotional security the 3-year-old child can separate from his parents for increasing periods of time; for instance, to attend nursery or playgroup.