what the ground, a rock, and my knee have in common

Contact, of course.

Some may say I'm too old for this. Others may envy my life. I do.
Was riding at the end of this week. Hit the Aspen Trail at a speed that was a speed I hadn't done in a while. Was just thinking of how cool it was to have the skill it takes to ride in tight single track and hit it fast. All of a sudden I had to decide to go around a rock and navigate the trail, taking a quick turn to the right. Started to jump the rock, took the particular turn a little faster than usual, clipped my bar on the edge of a tree and that was all she wrote.
The front wheel turn completely sideways and I hit the ground with a thud, slid about 6 feet and stopped just before my face was to hit a rock.

Dirt rash on the top side of the left arm, rash on the top side of the right arm and well, the left knee took the worst of it.

Oh well, that's riding. Ride and learn.