One strategy would be to focus on language.
The postmodernist is concerned with imprecision and unreliability of language and with epistemology, the study of what knowledge is.
An alien might come to earth and attempt to understand the planet by reading the literature of the planet, or just the dictionary. Looking up the word "earth" the alien may be surprised to see the this term has multiple meanings, refering to a planet and to a substance (soil/dirt).
The dictionary offers a firm set of definitions for this term, but no single defintion, which leads to a sense of complexity. The complexities of perception are, in part, what post-modernism is all about.