blind rivets (trademark: Pop rivets) have a center mandrel which is drawn out of the rivet, forcing the rivet to expand within the intended hole. The shank which is drawn out is then snapped off (abgeschert). However, part of this solid mandrel remains within the rivet body. It can be removed, but this weakens the rivet's strength. After removing the mandrel, the hollow rivet can be pounded flat or flush - necessary on aircraft fuselages, etc. so that the rivet does not protrude so far proud of, or above, the surface.