Electric Circuits

Electricity flows in a loop called a circuit. A circuit starts and stops at
the battery pack, and flows through wires, conductive dough, and electrical
components such as LEDs and motors.
Electricity is like water; it takes the path of least  resistance. It is easier for the electricity to flow through the dough than  through the LED, so if the dough on each side is touching, electricity does not  flow through the LED at all. Therefore, the light stays unlit. This is called a  short circuit.Instead of separating the pieces of dough, you can also use  the insulating dough to separate the conductive dough. Unlike conductors,  insulators do not electricity flow through them, so the electricity must go
through the LED.
The LED (Light Emitting Diode) produces light from electrical
power. To work, it has to be oriented properly (this is called polarity).
Usually the two leads are different lengths. The longer lead goes to the
positive, or red, side of the battery pack. The shorter goes to the negative, or
black, side of the battery pack.
You must have dough between the LED and battery terminals or else the LED
will burn out.

Check out this website for more ideas:
http://courseweb.stthomas.edu/apthomas/SquishyCircuits/howTo.htm