A ketchup bottle cap inspired Princeton engineers to build a shape-shifting robot that moves without a motor

At Princeton Engineering, a team led by Glaucio Paulino has answered a question few people ever think to ask: what makes a ketchup bottle cap snap open and stay open rather than flopping shut? The hinge on a typical flip-top ketchup cap attaches a thin, flexible shell to a thicker, rigid base along a curved edge, creating 2 distinct stable positions separated by an energy barrier that the cap must overcome to switch between them.

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