system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why as an adult sitting on your knees is so painful compared to when you were a kid.
gpt
That possition puts your weight on a relatively small area.

This matters more when you're an adult than when you're a kid because of the square-cube ratio.

The square-cube ratio means that while your volume and mass increase by a power of 3, your surface area only increases by a power of 2.

This means as your size increases, the ratio of surface area to mass decreases, and you therefore have more pounds per square inch on your knees.