twin trains

A few weeks ago Ezra and I had a conversation about what makes people twins.  I kept it simple and told him that people are twins because they grow in their mommy's tummy at the same time.  That was that, conversation over.  Until 2 days ago.  Out of the blue, Ezra asked me, "are trains born?"  I told him that trains are built but not born.  He looked thoughtful and said, "but they are born on the Island of Sodor."  I paused for a minute, not really sure what he was getting at but finally responded that those trains are built, too.  He looked confused and then said, "Well then how can Bill and Ben be twins?"