A bungee cord is an elastic cord composed pf one or more elastic strands forming a core.usually covered  in a woven nylon or cotton sheath.The sheath itself does not materially extend elastically,but it is braid with its strands spiralling around the core so that a longitudinal pull causes it to squeeze the core,transmitting the core's elastic compression to the longitudinal extension of the sheath and cord.Today,bungee cords are most often used to secure objects without tying knnots and to absorb shock.