Not a physical boundary!

As you all know, Charlie is the smartest dog ever. A while back we put up the flags to mark the boundary of our invisible fence and started training Charlie. Several sessions on the leash was really all he needed. Once he got the shock, he quickly learned not to cross the flags. Tonight was basically the final exam. AND HE ACED IT! We started by throwing the frisbee to him inside the yard. He ran like crazy after it and brought it back. Then I threw it outside fence and he didn't budge. Then as we were playing two kids and their two dogs came walking past. Normally this scene would end with Charlie either knocking over the kid in excitement, sniffing a lot of doggy butt, or just crazy bitey horseplay with the other dogs. Not tonight. He ran back and forth along the invisible boundary and didn't cross. Then as the final exam was coming to a close, I decided to have Charlie chase me. I ran from inside the yard down to the sidewalk. He chased me right to the boundary flags and then ran along the sidewalk while staying inside the flagged area the whole time! What a puppy! A+.