Let's take a sheet of paper and draw an arbitrary polygon. Is it possible to fold the sheet so that the polygon could be cut out with a single straight slit?
Consider the simplest case of an arbitrary triangle.
We draw a straight cut along it.
If you unfold the hole has the form of the initial triangle.
Let's draw a star.
It is also possible to cut out by one straight slit the polygon drawn in the beginning of the film. In 1998 the general theorem was proven.