Lullabelle
Midlands, England
OK, so when I kneel down whilst gardening, wearing trousers- how do I get muddy knees 

Can you order muddy knees on-line?
No, seriously. First, you need to prep the area. Take out your garden hose and thoroughly wet the area where you will kneel. If possible, spread some fresh dirt before hosing. Manure would be great. Then go ahead and kneel.
Not difficult. The "Mud" or silt particles that make it to the skin of your knees are much smaller particles than the weave of your trousers. trouser fabric or fabric of most kinds is actually quite "coarse" weave compared to the silt particles. Kind of like using a standard seive to make a nut based milk and wondering why not much is caught and why the milk is grainy.OK, maybe I should have been a bit clearer, why do I get muddy knees even though I am wearing trousers.
If I kneel down when I´m gardening, I never get up again.
I bought myself a cheap plastic stool, which works better for me.