Materials Failure Analysis: A broken jam jar was examined to determine the cause of the failure. In this case, the top of the jar separated from the body as the user was trying to twist the metal lid off. The upper left photo is of the severed top of the jar. The lid did not come off. There is an impact point on the glass at the bottom of the photo. In addition, there are two dents to the lid. The upper right photo is a 15X photomicrograph of the fracture surface at the failure origin (red arrow). One can see an impact failure with two crack arrest lines. This jar failed because it had been hit by a hard tool during an attempt to loosen the lid. The dents on the lid suggest that the jar had been hit several times. The root cause was one blow to the glass; it created the crack that was to cause the top to separate from the rest of the jar as the lid was being twisted.