There are a lot of youtube videos and online forum speculations that suggest that tossing a disassembled spark plug at a car window will cause the window to spontaneously shatter. The supposed justification is that tempered glass can easily withstand from softer materials than it (such as the iron of a hammer), but even a light brush with a material of higher Mohs scale strength (such as the Aluminum Oxide within the spark plug) causes it to spontaneously shatter in a manner similar to Prince Rupert's drops. Is this rumor true? If so, does someone have a more detailed explanation of the physics behind this effect. Sources would be excellent- google is not turning up any credible information. Also, what specific properties of car door glass make it susceptible to this effect? Thank you in advance.