What drives physicists to study cracks? There is certainly some attraction in being able to tell the children one is getting paid to break things. There is also a perverse pleasure in learning the physical laws underlying irreversible change, decay, and destruction. A paper of Eran Sharon, Steven Gross, and Jay Fineberg, "Energy Dissipation in Dynamic Fracture" which appeared on 18 March in Physical Review Letters contains an answer to an old and deceptively simple question, "How fast do things break, and why?"The first scientific attempts to answer this question go back to research of Hubert Schardin and Wolfgang Struth in 1937[1], who used sparks to take photographs of cracks in less than one ten-millionth of a second. They concluded that "the maximum velocity of propagation of glass fractures is to be considered a physical constant," and they measured crack speeds that were approximately one quarter the speed of sound in many different glasses.Some time after these experiments came theories of crack motion, which firmly insisted that cracks should move at about twice the speed observed. The classic theory[2], developed by B. V. Kostrov, J. D. Eshelby, and L. B. Freund, left little room for uncertainty. Cracks leave two new surfaces behind them. The speed at which vibrations travel across a free surface is the Rayleigh wave speed, a speed governing the motion of sound when one raps one's knuckles on the top of a table, or the speed at which earthquakes travel on the surface of the earth. Cracks, said the theory, should move at this Rayleigh wave speed too -but it does not happen. In Plexiglas, for example, the Rayleigh wave speed is around 1000 metres per second, but cracks never exceed 600 metres per second. This descrepancy was widely acknowledged to 1