Took a fall

I ran my usual 2.5-mile loop on the trails at the park this evening.  I tripped on a root, took a fall and cut my left knee in the usual place.  After falling, I realized that I tripped on the same root that I tripped on about 3 weeks ago and cut my knee, which just finished healing.  I wonder how many times over the years that I cut that knee in the same place.  It keeps on healing.  I also wonder if some point it will not heal anymore and I have to get a skin replacement.