After Lacan, Zizek has used lots of topology, especially shapes like the Klein bottle, torus and Mobius strip (see Sex & The Failed Absolute). Such “non-orientable shapes” are better described as surfaces where you cannot consistently distinguish inside from outside. In theory, if you move around them, directions flip. They can be used to metaphorically describe subjectivity and the psyche, where distinctions like inside/outside, self/Other, or appearance/reality collapse.

The joke is that any “responsible adult” would rejects this confusing, paradoxical thinking and prefer a simple model of the mind, with clear boundaries and stable categories (like: “I = I”, or “I am equal to myself”)

The image is based on an old Soviet anti-alcohol poster.

The Klein bottle definitely has an “inside”. Liquid affected by gravity will be at rest there until the bottle is reoriented, just like other bottles. The only difference is that the opening is curved and smooth.