Is it ensure, insure, or assure?
A knockout or a knockout punch (1940s+ Prizefight)
piece of rubber or other material used to rub out marks made by ink, pencil, or chalk. The modern eraser is usually a mixture of vegetable oil, fine pumice, and sulfur bonded with rubber. The mix is processed, extruded, and vulcanized in rubber-processing equipment. In 1752 the Proceedings of the French Academy reported a suggestion for using caoutchouc, a vegetable gum produced by certain South American trees, to erase black lead marks. Caoutchouc was named rubber in 1770 by the English chemist Joseph Priestley, because it was used to rub out marks. The first patent on an integral pencil and eraser was issued in the United States to Joseph Rechendorfer of New York City on March 30, 1858.