Impossible Stairs

The Penrose stairs, also known as the impossible staircase, is a two-dimensional depiction of a staircase in which the stairs make four 90-degree turns as they ascend or descend yet form a continuous loop. Although the staircase is conceptually impossible, it does interfere with your perception since it looks like a person could climb forever and never get any higher.

Watching the video is the easiest way to see how it works.
https://youtu.be/VAAZBhA8Pfc
Build Instructions: https://www.instructables.com/id/Impossible-Staircase/