Fooled by depth
Anamorphosis
What is the long smear at the bottom of this 1533 painting?
A skull: stand close to the painting on its right side and look along it.
Why? The painter, Hans Holbein, stretched the skull so it only looks right from one place, the way a shadow stretches in the evening.
Picture: Hans Holbein the Younger, Public domain · Wikipedia(opens in a new tab)