First Encounters of the Close Kind - S3-E10
Plot hole: Joey tells AJ that her Mother used to read "Little Women" to her, and in fact named her 'Josephine' because "Little Women" was her favorite book. But in a previous episode when Joey and Dawson visit her Mother's grave, the gravestone has printed 'Lillian Josephine Potter', meaning her Mother already had Josephine as her middle name. So naming Joey 'Josephine' wasn't only because of the book "Little Women"; her Mother more likely named her that because it was her middle name.
Answer: It's only explained once in the series. Jack says to Andie "I tried to start kindergarten without you", meaning Andie was smart enough to start school early.