Have you ever looked down at your pair of jeans and noticed that they’ve “developed” since you bought them? That you’ve got wear patterns where you put your keys and phone, where your knees bend, and *gasp* is that a blowout at the crotch?

Just like glory, denim fades, and it all has to do with the age-old battle between cotton and indigo. So stick around while we explain why jeans fade.

Indigo is one of the oldest dyestuffs known to man. We’ve been blueing ourselves for nearly five thousand years now. It started with the plant indigofera tinctora aka “natural indigo,” but most of what you find in denim today is synthetically generated in a lab and hence known as “pure indigo.”

Both are just awful, horrible dyestuffs from a technical point of view, but therein lies why we love them so much. Sort of like oil and water, indigo and cotton do not want to stick to each other. You can’t just put indigo in water and expect it to dye anything you dip inside.

The molecular composition of indigo is quite rigid and stable, so it doesn’t naturally adhere to a cotton fiber. The indigo has to be broken at that molecular level with other additives to form a dye bath, which then oxidizes back to rigid indigo when exposed to light and oxygen (see here for the full explanation).

It takes an intense amount of heat and water to get them to bond together on a commercial scale. But once the indigo is adhered, it reverts back to it’s rigid state and creates a brittle lattice on the outside of the fiber.

Cotton is most often dyed when it’s in a yarn form and before it has been woven into a fabric. Most commonly, the yarns running up and down (textile pros call these the warp) are dyed blue in the indigo and the ones running side to side (aka the weft) are left natural. Hence your jeans that are blue on the outside and white on the inside.

The warp yarns in vintage denim fabrics were dyed in a process called “rope-dyeing”, which takes all of the yarns together in long ropes and briefly dunks them in indigo. They then emerge before getting dunked again. This constant exposure and dunking means that they never sit in the indigo long enough for the dye to get all the way to the center core of the yarn, so the outside gets blue while the inside stays white.

Because indigo forms a brittle bond on top of the cotton, the two part ways with even the slightest encouragement. Mostly that comes from friction–where your jeans crease when you walk or sit down, where you stuff your wallet in your pocket, when you did that sick controlled slide on your fixie–all of it chips away at that outer indigo and exposes the white cotton core.

Fade devotees even have names for the different shapes and places they develop: You’ve got your whiskers along the lap because they look like they’re coming off a cat or something, honeycombs on the back of the knees because those fades often have a similar geometric pattern, and stacks down by the hem because they…stack.

Even though most jeans these days aren’t rope-dyed—they’re stuffed in a big vat where the dye gets closer to the core—they’ll still fade. But rope-dyed raw denim is most likely to give you high contrast (read: sick) fades and the method preferred by many raw denim fans.

So the next time you see that guy at the hardware store with the worn white circle of Skoal in back pocket, know that it’s just another casualty in the endless war between cotton and indigo.