I've been looking all morning for a short story or essay I read in high school. The writing was overly detailed and abstract describing these 'things' you thought to be aliens by their odd behavior, but the end reveals they are actually humans.
Possibly Body Ritual Among the Nacirema
Story about an anthropologist studying strange beings who turn out to be human, it's a common read in high school and intro uni courses.
The focal point of the shrine is a box or chest which is built into the wall. In this chest are kept the many charms and magical potions without which no native believes he could live. These preparations are secured from a variety of specialized practitioners. The most powerful of these are the medicine men, whose assistance must be rewarded with substantial gifts. However, the medicine men do not provide the curative potions for their clients, but decide what the ingredients should be and then write them down in an ancient and secret language. This writing is understood only by the medicine men and by the herbalists who, for another gift, provide the required charm.
You can read it here.