People in development agencies like milestones - not the real life objects, but those imaginary markers which indicate that a project is progressing as planned - or not. Craftspeople, engineers, cooks and other people who work with tangible objects know what milestones they need to pass on the way to the finished product. Sometimes traditional ceremonies accompany the passing of such milestones - for example, in Germany people have a ceremony when they complete the roof structure of a house.
