Physicists at the University of Pennsylvania have experimentally discovered a fundamental principal about how solid materials melt. Their studies have shown explicitly that melting begins at defects within the crystalline structure of solid matter, beginning along the cracks, grain boundaries and dislocations that are present in the otherwise orderly array of atoms. Their findings, which will appear today in the journal Science, answer longstanding fundamental questions about melting and will likely influence research in physics, chemistry, materials science and engineering, as well as studies of biological importance.