Good evening new Backyard Chicken friends.  I inadvertently ran a little experiment today and wanted to share my results, from which I have drawn my own conclusions.  First, when I arrived home from the grocery I boiled 15 eggs, 9 of which were from a partial carton I already had in the fridge.  I had purchased these 9 eggs a week ago from a very large superstore.  The additional 6 eggs were from a small chain grocery-only store from which I purchased the additional eggs today.  I typically boil my eggs in a stainless steel pot and have recently started adding vinegar to the mix in order to "soften" the shells and ease peeling.  Today, I boiled the eggs in a cast iron pot and added apple cider vinegar.  Some amount of spotting was visible on 12 of the eggs and therefore I know it wasn't related to either individual carton.