If you tap the back of your head, you will find it is very hard. But in early infancy it is not quite as tough and with the right techniques, it can be moulded into a different shape, changing how it looks forever.
Welcome to the practice of reshaping the human skull. It has been documented in groups around the world, 45,000 years ago until today.
Some did it for aesthetics, some for power. In many cases, we can only guess why people did it.
It now turns out that skull-reshaping was common practice among hunter-gatherers in Patagonia, South America 2,000 years ago. Dozens of their reshaped skulls are offering intriguing new insights into why they did it.
In 2009 a team of archaeologists found intreaguing remains at an ancient burial. Marta Alfonso-Durruty, an anthropologist at Kansas State University in Manhattan, US, was invited to Chile's Instituto de la Patagonia to investigate them. She had been told to expect one extremely modified skull.
A skull modifier has to start early in childhood, when a child's skull is still soft enough to have its shape altered
When she arrived and starting looking at the other skulls housed there, she found there were actually several modified skulls, something not previously observed in Patagonia. Out of 60 adult skulls, 18 (30%) were modified.
It seems likely that examining more skulls would reveal many more instances of modification. Aside from the obvious one, nobody else noticed the modifications, because the other changes were subtler and could look like natural variation to the untrained eye.
When it became apparent just how many had been modified, Alfonso-Durruty and her colleagues began to examine more skulls in a bid to figure out why.
Modifying a skull is not easy. Our skulls are quite tough, so by the time they are fully formed it is already too late to change them.
The back looks like somebody slammed a door on it
Instead a skull modifier has to start early in childhood, when a child's skull is still soft enough to have its shape altered. The decision to reshape it would have been made by a child's carers, likely starting shortly after they were born.
This should go without saying, but do not try this at home.
One way a parent might reshape their child's skull is to bind their head tightly with bands of cloth. This forces the skull upwards and gives the head a cylindrical shape, pictured above.
However this method is dangerous: if the cloth was wrapped too tightly it could kill the child, as was the case for some Peruvian remains described in 2008.
Another method created a different shape. Flat-headed skulls (below) were created with hard wooden boards. A baby's head would be pressed against these for long periods of time, sometimes at the front and back. "The back looks like somebody slammed a door on it," says Alfonso-Durruty.
For example, some Native Americans used a cradle contraption.
It can serve as a permanent symbol of within-group solidarity
"The baby was tied to the cradle so the mothers would not have to worry about them. That sort of deformation could have been (at least initially) accidental," says Mercedes Okumura, an archaeologist at the National Museum, Federal University of Rio de Janeiro in Brazil. "After some time, people realised that the baby's head would flatten and so they would begin to use the wood cradle in order to get an intentionally deformed skull."
The transformation is irreversible, and to many people it will sound extreme. But cranial deformation was an important way to identify with a group. It could also be considered beautiful, says Okumura.
"There is strong evidence that physical manipulation of skulls was undertaken not just to reinforce social distinctions, but also to entrench political power. Therefore, it can serve as a permanent symbol of within-group solidarity and of cultural differences between groups."
In some cases cranial modifications served as a status symbol that signalled a person's social class: this was the case for the Oruro people in Bolivia. "High-class individuals had tabular erect heads, middle-class had tabular oblique heads, and the rest of them had ring-shaped heads," says Okumura.
The best way to expand is to make friends with others
But the Patagonian hunter-gatherers did not live in well-structured societies where such distinctions needed to be highlighted. Instead, they spent their lives migrating from place to place. So Alfonso-Durruty was surprised that so many of them had reshaped skulls.
They probably did not reshape their skulls to show group membership, she says, but to help them expand their territories and gain access to new resources. She and her colleagues outline these ideas in a new paper in the American Journal of Physical Anthropology.
That may sound strange. How could an odd-shaped skull help groups gain access to new places? When you begin to unpick the details, it starts to make more sense.
These people lived in areas with patchy resources, says Alfonso-Durruty. "In that case your best strategy is to expand your network, so that you and your group can have access to different patches along the territory."
The best way to expand is to make friends with others. Because reshaping your skull is not an easy thing do, or fake, those that did so immediately showed that they were a trusted group. "By doing so it would signal relationships with individuals that are in other regions," says Alfonso-Durruty. Simple marks on their bodies would not be such a strong signal.
We do not know how long the skull reshaping went on for
It paid these people to be well connected, and having an oddly-shaped skull proved they had acquired information on how to do so from another trusted group. "It was a social strategy that allowed individuals to access resources, that were at times unpredictable, across a large territory," Alfonso-Durruty says.
It might even have helped their groups expand. The area is known to have experienced a population surge around this time.
The idea is also backed up by the team's analyses of what these people ate. Everything we eat leaves tiny traces in our bones, so by analysing their remains, researchers discovered these Patagonians had a mixed diet. They ate both land-based and marine foods, showing that they lived in different areas.
There are still many unanswered questions.
For instance, we do not know how long the skull reshaping went on for, or if it was more extreme in males or females. We also do not know which type of modification was preferred, or why.
We do know that the babies had no choice in the matter. But when you consider how many people today are willing to drastically modify their own bodies, it does not seem so surprising that some parents decided to mould their children's skulls, especially if it gave them a better chance of prospering as adults.
Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.
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The Patagonian remains are housed at the Centro de Estudios del Hombre Austral, Instituto de la Patagonia, Universidad de Magallanes, Chile.