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First, check whether the initiation point (the crack of tempered glass is radial and has a starting point) is in the middle of the glass. For example, if it is at the edge of the glass, it is generally because the glass has not been chamfered and polished or the glass edge is damaged, resulting in stress concentration and the gradual development of the crack; If the initiation point is in the middle of the glass, check whether there is a pattern similar to two butterfly wings (Butterfly spots) composed of two small polygons at the initiation point. If there are black small particles (nickel sulfide stone) visible to the naked eye on the common edge of the two small polygons (the trunk of the butterfly), it can be judged that it is self explosion; Otherwise, it will be damaged by external force. The typical feature of glass self explosion is butterfly spots. The glass fragments are distributed radially, and there are two pieces of glass in the radial center that look like butterfly wings, commonly known as "butterfly spots". NIS stone is located at the interface of two "butterfly spots".