when my daughter was five and began asking questions, so she learned about the mechanics at an early age. Answering questions honestly and tailored to age (not too much detail at first) and bringing in the rest as they grow older seems natural enough. I later lost the baby so she learned about death at an early age as well, an even more taboo subject I would say. It is surprising that even quite young children can cope very well with these subjects if handled sensitively and honestly.

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"There's this thing called being so open-minded your brains drop out". - Richard Dawkins