* Perspective: Escher was fascinated by the way that objects appear to change shape and size as they move closer to or further away from the viewer. He often used perspective tricks to create optical illusions.
* Relativity: Escher's works often explore the relationship between the viewer and the artwork. He would sometimes create works that could be viewed from multiple perspectives, or that seemed to change as the viewer moved around them.
* Nature: Escher was inspired by the natural world, and many of his works feature plants, animals, and landscapes.
* Architecture: Escher was also interested in architecture, and many of his works feature buildings and other structures.