Agree with sled re: dangerous extendables. I had a 50 footer for one collie I had. The problem is, if he gets a scent and you don't have your recall and he heads off at 10 ft, by the time he gets to 50ft, he's at full pelt, hits a full stop and turns into a catherine wheel. This can quite easily break his neck. Two people at either end of the garden with great treats, lots of exciting fun, different voices to his name, and play recall with your dog in turns. It can be fun for humans: just pretend you're playing that rubbish retro toy from the 70's where an organge plastic rugby-ball shaped thing had two long cords threaded through it with handles either end that each human pulled like a chest expander when it was coming their way.