A bungee cord is an elastic cord composed of one or more elastic strands forming a core ,usually covered in a woven nylon  or cotton 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 ,fransmitting 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.