I have a feeling I know how he’s doing it (this is his second scraped lip).  Our neighbors, on occasion, let their dog run free. He runs up to the outside of our wooden fence and races back and forth. Nina and Buddy just about kill themselves running back and forth with him. Buddy puts his face very close to the fence to keep an eye on the interloper, and I think he scrapes it when he bolts to run with the neighbor dog.

You can’t see it from this angle, but it actually extends up and sideways under his right nostril. It looks like it hurts…