A block of material with a crack in the middle is subjected to a tensile load. As the material is stretched, elastic energy is stored, represented in the graphic as a change in color. Crack propagation becomes thermodynamically favorable after the Griffith criterion is satisfied. The criterion states that the elastic energy released per crack extension is equal to the surface energy necessary to form the new surfaces. Also shown is a contour plot of the stress field, which diverges at the crack tip. Because the Demonstration is based on thermodynamics, time is not a parameter. Adjusting the stress, initial block height, surface energy, and Young's modulus changes the critical stress for the Griffith criterion. All units are arbitrary.