Drawdio is a simple but really cool electronic circuit designed by a guy called Jay silver.
It's an sound generator where the frequency is controlled by de resistence of any material you connect at it. Even yout body! It's a simple and fun machine to produce cool sounds.
The original circuit is attached to a pencil. Since graphite conducts electricity, the sound frequency will change according the tickness of your drawing. You can also use the resistence of you body, water taps and other electricity-conducting materials to generate sounds
Check this video: A piano I drew with Drawdio
There are several versions of Drawdio around the internet. You ca find some of them in the links at the end of this page.