I've found that it is mainly people in Michigan, Ohio, Indiana, West Virginia, Kentucky and the western parts of Pennsylvania that call it pop. I'm sure people in other places do too, but these are the places I've found it most common. I visited around the New England area and noticed everyone calls it soda. I also lived in south east Florida for awhile and soda seemed to be the most common name. I haven't been out west but I noticed on the show Little People Big World, that takes place in Oregon, they call it soda. So I think it's a regional thing.