What is a Klein bottle

Klein bottle, in the field of mathematics, refers to a plane without orientation, such as a two-dimensional plane, there is no "inner" and "outer" distinction. A Klein flask is an unorientable topological space in topology. The Klein bottle was originally proposed by the German geometry master Felix Klein.
In 1882, the famous mathematician Felix Klein discovered the famous "bottle" that was later named after him.
The structure of a Klein bottle can be described as: a bottle with a hole in the bottom now extends the neck of the bottle and twists into the bottle and then connects with the hole in the bottom. Unlike the cups we usually use to drink water, this object has no "edges" and its surface doesn't end. Unlike a sphere, a fly can fly directly from the inside of the bottle to the outside without going through the surface, i.e. it has no distinction between inside and outside.