This story is a sequel to The Birds and the Changelings
When a colt gets his answer to "where do babies come from?" he's fine. He doesn't need any more answers on the topic... Except when he gets pulled into another world. In that case, he wonders where human babies come from. But, by now the mom is burnt out on the question, and the dad didn't want to answer in the first place. So, it's up to someone native... The sister.
A companion piece to Know Your Mare.