Shrunken heads are a key part of the “scary tribal people” setup. And some cultures did, in fact, create miniature heads for religious and spiritual purposes. But how does one take a regular sized human skull and miniaturize it?

The process is gruesome, according to Today I Found Out. First, the skin and hair had to be separated from the skull to allow them to shrink at different rates. Then, the eyelids were sewn shut and the mouth was stuck closed with a peg. And for the actual shrinking, the heads were put in a big pot and boiled for a very specific amount of time. Then, Staci Lehman writes:

Once removed from the pot, the head would be about 1/3 its original size and the skin dark and rubbery. The skin would then be turned inside out and any leftover flesh scraped off with a knife. The scraped skin was then turned with the proper side out again and the slit in the rear sewn together. The process wasn’t done yet. The head was shrunk even further by inserting hot stones and sand to make it contract from the inside. This also “tanned” the inside, like tanning an animal hide, in order to preserve it. Once the head reached the desired size and was full of small stones and sand, more hot stones would be applied to the outside of the face to seal and shape the features. The skin was rubbed with charcoal ash to darken it, and as tribesmen believed, to keep the avenging soul from seeping out. The finished product was hung over a fire to harden and blacken, then the wooden pegs in the lips pulled out and replaced with string to lash them together.

When Westerners and Europeans started traveling and discovering cultures that practiced head shrinking, they were both terrified and fascinated. Many of them brought back shrunken heads and souvenirs. In the 1930s, a shrunken head sold for $25—$330 in today’s dollars. In fact, they were popular and lucrative enough that unscrupulous head-peddlers started trading in fake shrunken heads, made from the heads of sloths and other animals. And telling the difference between a real and fake shrunken head can be hard. In fact, one researcher claims that most shrunken heads on display at museums (including the American Museum of Natural History) are fake. Forensic researchers write about some of the ways to tell:

Tsantsas, or shrunken head, are an ancient traditional technique of the Jivaro Indians from Northern Peru and Southern Ecuador. Tsantsas were made from enemies’ heads cut on the battlefield. Then, during spiritual ceremonies, enemies’ heads were carefully reduced through boiling and heating, in the attempt to lock the enemy’s spirit and protect the killers from spiritual revenge. However, forgers have made fake tsantsas out of sloth heads, selling them as curios to international travelers. Morphologic criteria can help in the distinction of forged and authentic tsantsas. Presence of sealed eyelids, pierced lips with strings sealing the mouth, shiny black skin, a posterior sewn incision, long glossy black hair, and lateral head compression are characteristic of authentic tsantsas. On the other hand, fake tsantsas usually present few or none of those criteria. To establish authenticity of the shrunken head, we used all of the above-mentioned morphologic criteria along with microscopic hair examination and DNA analysis.

If you don’t have a DNA sequencer handy to identify your human head, William Jamieson Tribal Art says to look at the ears:

Imitation tsantsa are classified under two categories, being either non-human or human but prepared by someone other than the Jivaro tribesmen. As the most common non-human fakes are often made out of goat or monkey skin, one must pay particular attention to distinguishing between authentic and replicas. Indications of counterfeit tsantsa are characterized by looking for nasal hairs which is a notable distinction between identifying authentic heads and non-human replicas. In addition to this, it is also quite difficult to duplicate a shrunken human ear. The ear should remain in its original form only smaller. Fakes generally cannot match the intricate details of the human ear.

As for many topics of cultural anthropology in which the culture in question still exists and its members would like to be treated as people, head shrinking is a bit contentious. In the Shuar culture, shrunken heads (or ”tsantsas”) are extremely important religious symbols. One anthropologist writes:

That Shuar have killed people to make powerful objects, whereas we have made powerful objects to kill people, does not sustain any meaningful distinction between the savage and the civilized.

Is is hard for many people to not see shrinking of heads as a gruesome act. (Shrunken heads were found in the German concentration camp at Buchenwald, but never identified.) And many say that no new shrunken heads have been made for twenty years. In South America, many countries outlawed selling human heads in the 1930s. Whether or not heads have been shrunk since is still up for debate, but at least now you know how it happens.

More from Smithsonian.com:

An Evolving Ritual

Egyptian Mummification Rituals Uncovered at Natural History