One just a little bit up on that. When kids we would attach the cotton, ( not thread because that could be dangerous ), to as many door knockers as the cotton would reach,then, each time a door opened another knocker was activated. Note:I did live in a cul de sac and traffic was very few and far between in those days.....1940's. it didn't take long before we were caught though and grounded during the dark evenings.