A string telephone!
My son was playing with a cheap toy walkie-talkie and it wasn’t working very well. So I thought – what about a good old string telephone?!
I used two plastic cups attached with some string (and large knots within the cups). We then stood such that the string was under tension (i.e. taut) and took it in turns to speak into and listen to the cups. My son thought this was amazing!
We talked about how when you speak, your voice makes the string vibrate (I showed him what a vibration was by making a large vibration with the string), and that this then is heard as sound in your ear. We talked about sound waves travelling down the string. We played with this for about 1/2 hour and my son couldn’t get over how clever it was!