Bologna bottle

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A Bologna bottle, often used in physics demonstrations and magic tricks, is a glass bottle which has great external strength,

The exterior is generally strong enough that one could pound a nail into a block of wood using the bottle as a hammer, however even a small scratch on the interior would cause it to crumble.

It is created by heating a glass bottle then slowly cooling the outside whilst rapidly cooling the inside. This causes the external strength and internal stress such that even a scratch on the inside is sufficient to shatter the bottle.