The middleness concept in children.
D002648:Child; D002657:Child Development; D002675:Child, Preschool; D003071:Cognition; D003210:Concept Formation; D004193:Discrimination Learning; D005260:Female; D006801:Humans; D008297:Male; D009949:Orientation; D011340:Problem Solving; D012737:Sex Factors; D012858:Size Perception