Folds in the skin have always been ignored and denied of any importance.
People (including scientists) call them lines or wrinkles and fail to understand that the skin is actually folded.

What are those deep grooves in this baby's foot?

The word «lines» doesn't fit.
Surely those are more than just lines; they have a depth and lots of details along their path.
Calling them lines is a simplistic way of describing them.
Are they «wrinkles»?
Most people would say they are not, because wrinkles are characteristic of old and damaged skin.

Close visual and manual inspection only confirms that the skin is folded inwards.
You can trace the origin of these folds back to all the movements, expressions and positions you've taken, made or held during your life.
In many places, their path has tightened and solidified resulting in permanent folds.

The skin is so badly bent that it doubles up, and that one part now lies on another.
A section of skin is imprisoned.
Epidermis over epidermis.

Skin is a tissue.
Just like any tissue, it can be crumpled, folded and creased.

Simply press your nail onto the bottom of any fold in your hand.
Put moderate pressure and explore.
You'll feel that your nail can open up the bottom of the folds and that the folding extends inwards.

Your skin is a living tissue.
It contains blood vessels, nerves, hair follicles, sweat glands, touch, pressure, heat and pain receptors, ...
Skin is very elastic; it needs to because your movements deform it considerably.
It can withstand repetitive folding, but there is a limit to what it can endure. 

One can marvel that such a complex and delicate structure can be pinched and folded so easily.
However, when your skin is folded too often, it fears rupture and it paves the bottom of the furrow with epidermis cells, making the fold permanent.

Cutaneous folds are the main cause of acute pain in every location on the body.
Pain in the shoulders, in the arms, in the hands, in the back, in the knees, in the feet, ... they all have the same cause; folds in the skin.
However, the pain is mostly concentrated in the places where two folds meet; the folds crossings.
● More on cutaneous folds and pain.

Cutaneous folds bend the nerves that pass beneath them and weaken the signal transmitted to the brain.
Each nerve travels from a specific part of your body to your brain.
However, they are susceptible to being bent, compressed or pinched by the folds in the skin along the way.
When the strength of the signal the nerves transmit is diminished, you slowly lose contact with some body parts or functions.
● More on cutaneous folds and loss of sensitivity.

Cutaneous folds bend, compress and strangle the blood vessels and veins beneath them.
They are the main cause of hypertension and reduced blood circulation.
Folds are often the cause of blocked blood vessels and arteries (atherosclerosis), high blood pressure (hypertension), numbness and tingling (paresthesia), varicose veins, and more.
● More on cutaneous folds and blood circulation.

When you see aged skin, you see folded skin.
In fact, the folds are responsible for the most disturbing aspects of elderly life such as: chronic pain, loss of mobility, circulatory problems, loss of sensitivity, ugliness, ...
● More on cutaneous folds and aging.

Ugliness is not a health problem, but it can have a negative impact on someone's social and sexual integration.
● More on cutaneous folds and beauty.

Flex your wrist several times and take a close look at the folding that this action creates.

Notice that some folds do not disappear completely when the articulation is straight.
These folds seem etched into your body because you have done this movement so frequently throughout your life.
Those are permanent folds.

When the folding in the skin occurs at the same place repeatedly, your system generates epidermis cells to pave the bottom of the groove of the fold.

Of course, when a fold becomes permanent, the trajectory and placement of its components becomes altered and this affects their performance.

When I first found out about the folds, I thought that each human was folded differently, explaining the diversity of our looks.
Well, I was half wrong.
In fact, most folds are common and comparable from one human to another.
We look different because some folds have a slightly different position and depth, but mostly because of our bone structure beneath.
The similarities are much greater than the differences.
We are all folded the same way.

Differences often lie in the size, angle and depth the same fold can take among humans.
Early in life, the shape of the skull is very important, the alignment of its parts particularly.
For example, the fold going from the side of the eye to the top of the ears is obviously affected by their respective positions.
Most of the distinctions come from how big the fold has become along its path.
This, in turn, depends on each individual's actions.
The way your nose is twisted, the way your eyes look, the way your eyebrows are formed, ... represent years of nose twitching, eye squinting or worried facial expressions.

Folds are illustrated here as simple lines.
They're not so on your body.
Each fold is a complex monster, full of details.
They're made of several coats, each one with a real-life history.
A typical fold has hundreds of coats.
They're also far from straight.
Folds multiply, so you may find several small folds along with one big one when you check on your body.

A wrinkle is simply the first step in the formation of a permanent fold.
With time, a wrinkle will become a fold.
Every large fold on your body has started out as a wrinkle.
When permanent folds first establish themselves, they pave the bottom of the crease, layer by layer, with epidermis cells.
It may take several years before the fold to becomes plainly visible.

Three main reasons explain why your sight is deceived;

Your skin considers folds and their crossings as injuries and tries to repair the damage.
The wound healing process coats every hole and crack with fresh epidermis cells to preserve the skin's integrity.

Folds and their crossings keep on growing and become huge intra-cutaneous structures.
They do this by deepening and bringing more and more skin into them.
Their entire mass is beneath the surface and from the outside, all you can see is a simple seam.

Most of the folding is hard to distinguish, because it's made of epidermis over epidermis.
A very tight seam, with no stitches, with perfect color matching is not easy to detect.
Even a microscope would only reveal patterns of epidermis cells.
To your ultra-sensitive nails and fingertips though, the folds are like mountains and canyons.