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In this book, Mary Douglas gives a cool explanation about why some things are taboo. She says that things are made taboo when they don't fit into the classification system that a society has adopted. For instance, she says that eating pigs is forbidden by some religions because the pig, a cloven-hoofed animal, does not chew it's cud like other cloven hoofed animals. This makes it hard to pin down and therefore scary.
This is an extension of the important rule: "If it's werid, don't eat it."
Looking at the context of a given taboo, we can find out what makes it scary, and gain insight into the society's classification system.
This concept is linked to things such as liminality, where the initiate is bewteen categories and therefore becomes taboo.