Why a stone arch stands without glue: Hooke's upside-down rope
30seconds
A stone arch has no glue, yet it stands for centuries because every block is only ever squeezed. In 1675 Robert Hooke hid the secret in an anagram: as a hanging cord, so, inverted, stands the rigid arch.
Hang a string with weights, drag them heavier, then flip it upside down. Pulls become pushes, and the same curve is a shape that stone can hold. See why flat arches shove sideways and tall ones don't, and why Gaudí built upside-down models.
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