Record which of the materials transmits the sound. Place it against your ear. Have someone hold the watch against the bag of water. Plug one ear, and place your other ear below the water surface at the other end of the tub. At the pool you might have someone bang two objects together underwater at one end while you listen with your head underwater to receive the sound.
Students become involved in learning through investigation which include many hands-on experiences.
Use a small, sturdy cardboard box. Cut away one side of the box almost to the edges but leaving a small triangular area near each corner to preserve the strength of the box. Using different widths and lengths of elastics, stretch them around the box and across the cut-out section. Pluck the elastics. Vary the tautness of the elastics.