My friend has a deceptively simple answer to this question: If you put schmear/spread/sauce on the bread, it's a sandwich. If you put schmear/spread/sauce on

My friend has a deceptively simple answer to this question: If you put schmear/spread/sauce on the bread, it's a sandwich. If you put schmear/spread/sauce on the "meat", it's not a sandwich. The distinction is that in the latter, the bread is just a vessel to get "meat" into your mouth. In the former, the bread is part of the whole package.