A Klein Bottle, named for Felix Klein, is a single sided bottle with no boundary. Its inside is its outside. It contains itself.
Of course, such a thing cannot truly exist in our 3 dimensional world. Still, with a bit of ingenuity it is possible to create something that looks like a Klein Bottle.
The red handle connects from the white inner bag to the outer blue bag.
Here’s how a net bag and a Klein bottle merged together to create a net Klein-bottle Bag.
Buy a net Klein-bottle bag or instructions.