It's a 7" long x 2-1/2" dia PVC tube with a resonant head at one end attached to a 17" long x 3/16" dia extension spring. Kids shake it, scrape it, wiggle it, use their palm to muffle it and they generate creaks, deep warbles, rumbles, weird heterodyne-ish gong effects, wa-wa's, rude noises, and, yes, realistic thunder. A perfect musical companion to a cuica and rain stick - and good, clean fun around the house.