Sexual curiosity normally begins during the third year, in the shape of an interest in the physical differences between men and women, and between adults and children. By nature this curiosity has no special quality in early childhood, but is simply a part of general curiosity. The special quality which it is found to have in children who are being conventionally brought up is due to the grown-up practice of making mysteries. When there is no mystery, the curiosity dies down as soon as it is satisfied.
Source: On Education, especially in early childhood, 1926, Pt. 2： Education of character, chap. 12: Sex Education
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