Billy has accidentally thrown his ball into the neighbor's yard!
The ball landed $15$ feet from the big oak tree in their large front yard. The trouble is, the neighbors have a mean dog that's on a $30$–foot long chain bolted to the tree.
The dog is faster than Billy and quite aggressive, so it always positions itself to minimize the distance between the two and will attack if possible.
Billy comes up with a plan to retrieve his ball that requires no special equipment, and within minutes, has accomplished his goal. How did Billy retrieve his ball without getting bit?