I know this question is the perfect setup for innuendo-laden comments, but it is a serious question, so please limit responses accordingly.
There are at least a few activities where my natural inclination may be to kneel (e.g. sitting on the floor to play with my nieces/nephews, rotating tires on my car, working on appliances).
It seems like children can sit on their knees all day long. However, any time I have seen anyone over the age of 20 get up from kneeling to talk/play with children on the floor (myself included), they say "I'm not as young as I used to be" implying some degree of stiffness/knee pain.
Short of knee pads, is there an exercise or activity that can done to significantly improve the ability to kneel for extended periods of time without pain? Or, is this really just an unnatural position for humans that requires additional padding?