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The glass broke by first spreading cracks through the glass, then seemingly simultaneously shards flew apart from the whole glass. I'm curious what accounts for the sharding part appearing simultaneous. Why didn't the sharding process start on one end and travel through the glass?

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That was a piece of tempered glass, as we can tell due to the way it broke into tiny pieces. Due to the tempering process, where it's heated then quenched by large fans, the glass is under stress that's concentrated at the edges, and especially the corners.

This is why it explodes when they break the edge the way they did. Ordinary annealed glass would've broken into large, sharp shards, whereas laminated glass would form a break pattern like annealed, but would stay in one piece due to being attached to the plastic inner layer.

They speed up the second part of the video. The "flying apart" bit happens at much lower speeds and would be boring to watch in extreme slow-motion.

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