The Magnetic Shaker is compact, easy to use, and a must-have device when scuba diving. Clip this device onto the B.C.D. so it's handy when it is needed to get the diving buddy's attention. A magnet is located inside on either end of the shaker with a ball bearing in between. Shaking the device moves the ball bearing, creating a loud echo sound underwater. It's a great communication system in a world where words simply won't be heard.