I tripped in the parking lot at work today, very ungraciously, and fell hard on my left knee. My skin is very scraped up (to the point of bleeding), but my thin cotton dress pants don’t have so much as a pull. This has happened before, most recently when I was wearing jeans at the ballpark and scraped my knee on some concrete but did absolutely no damage to the denim.
How is that possible? How could my knee be scraped and cut but the cloth covering it be pristine? 
You scraped your knee on the fabric, not on the ground. There was more friction on the skin to fabric connection than the fabric to ground connection.
Oh, that is a really bright answer. Thanks - I’ve had a similar scenario happen to me. I was in denim, but the same thing only worse - long bloody scrape on my knee, jeans were a bit dusty.
Thanks! It still seems very odd to me that the pavement would do less damage to the thin cotton than the thin cotton did to my skin. 
You got a brush-burn. The force of your knee hitting the pavement probably caused the material of your pants to slide along your skin.
There was actually probably more friction in the fabric to ground interface. Enough to lock it in place so it didn’t move.
I did this many times as a clumsy child. My mom would freak when I got undressed for my bath and my entire shin was covered in dried blood.
(I loved freaking out my mom. :D:D)
You might have left a good bit of skin on the inside of your pants leg. I did the same thing once, and then peeled away a quarter-sized piece of skin that used to belong to my kneecap from the inside of my jeans. It made a creepy noise when I peeled it off too. A very memorable experience.
I put the pants in the laundry basket when I got home, but did not check the inside of them for skin (or damage). Given the severity of the scrape on my knee, I think I will be checking those pants when I get home. Good tip, thanks. 