As far as I can tell, Squishy Circuits is originally from the Playful Learning Lab at the University of St. Thomas. The site provides recipes for conductive and insulating dough, and project ideas for all different levels. In exploring more while prepping to do Squishy Circuits with our 4-H club (last October, can be seen here) I discovered that Play-doh will serve as conductive dough. That's right, regular old Play-doh in the bright yellow can conducts electricity. Mainly because it's basically made of flour, salt and water. Salt water conducts electricity quite well.
When we did the project with our 4-H club, I made insulating dough using the recipe at the University of St. Thomas link above. Basically this dough is made from flour, water and sugar. But, at Science-Sational Day we were going to be doing 3 sessions with 15 kids in each session. And the dough would probably not be reusable from session to session. Since the kids were not going to be able to take their projects home (to expensive with the batteries, battery packs and LED lights), we decided to let them explore some other insulating materials.
We started with a presentation using some of the background materials available at the University of St. Thomas link, giving the kids information on electrical circuits, serial circuits, parallel circuits, and closed/open loops.
We then gave the kids a variety of materials to explore. We provided paper clips (conducting), Legos (insulating), Play-doh (conducting), popsicle sticks (insulating if dry), straws (insulating), index cards (insulating), Aluminum foil (weak conductor).
In order for the LED's to light, there must be insulating materials between the conductive clay. Otherwise the electricity will take the "path of least resistance" and travel through the clay, rather than do the work to light the LED.
George demonstrated the various materials during the presentation.
The kids were then able to come up with their own projects. All of them were able to get a simple circuit to work. After that, they came up with all kinds of elaborate creations.