This three-pronged fork looks fine up close, but count the prongs and it falls apart
The blivet, also called the devil's fork or impossible trident, shows two parallel prongs at the base smoothly turning into three rounded prongs at the top. Follow any single part of the drawing and it looks locally consistent. So why does the whole image refuse to resolve into a real, buildable object?
Reveal the answer
Each end of the figure is individually a valid 2D projection of a 3D shape, but the two interpretations contradict each other once your eye tries to reconcile them into one object, exploiting how human vision infers 3D structure from local cues rather than checking global consistency. Versions of the illusion circulated in US engineering-trade and student-humor publications from the 1960s, and it became a staple of optical-illusion collections.
— Unknown; popularized in US engineering/trade publications, Blivet — Circulated from the 1960s