Rosie's Fastener is a RIVET, which is a type of mechanical fastener used to join two or more pieces of material together. It consists of a smooth cylindrical shaft with a head on one end. The other end is inserted into a hole in the parts to be joined and then the head is hammered or pressed down to form a second head on the other side. Rivets are commonly used in the construction of aircraft, ships, and other structures.