This is part of a series of Instructables intended for teachers about educating students in the classroom around making and tinkering. For more about the details of this project, check out this video on YouTube.

Squishy Circuits is an award-winning activity developed by Dr. AnnMarie Thomas that introduces students to many fundamental concepts concerning circuits and electricity without the complications that come with traditional circuitry components. I have successfully facilitated this activity with kids as young as 4 or 5 years old - a true testament to just how accessible it is.

At its core, Squishy Circuits is all about learning the difference between conductors and insulators as they relate to electricity. Follow along and learn all about how to make circuits with play-doh!

Step 1: Materials Needed

This is a pretty simple activity in terms of supplies. You likely have most of the items or suitable substitutes kicking around your house, except the LEDs.

Play-doh (homemade or store-bought both work). AnnMarie Thomas has the recipe here for the conductive dough. I have not made the insulating dough, I've heard it tends to end up being rather messy and sticky, but YMMV. Commercially available dough - even the knockoff versions of Play-Doh - tends to work just fine in my experience.
9-Volt batteries
Alligator clips or wires or 9V battery snaps
LEDs - I prefer the larger 10mm variety found here, any regular 3V LED will work though
Paper, craft foam, wood, plastic, etc. Anything that is an electrical insulator will work
Pipe cleaners or insulated wire (16-20 AWG would be fine)
Optional: 3V piezo buzzers, vibrating motors, or any other exciting component that is safe which operates at 3V
Optional: I find it helpful to solder and/or crimp male disconnect terminals to the leads of components like the battery snaps, motor and buzzer wires.

Step 2: Hook Up the Battery

Once you've gathered your materials, get out some play-doh and make two balls about the size of a golf ball. Attach either two alligator clips to the battery or the battery snap. Pay attention to which clip is the positive and which is the negative - polarity is very important for these circuits! Once your battery is hooked up, stick one of the leads into one of the balls of dough, and the other in another ball. You have a partial circuit here, but we need to close the loop by adding a component.

Step 3: Safety! and Plug in an LED

Before we do this step, there are two important safety considerations to address with this activity. The first is that if you short the 9V battery for too long, it will overheat and start smoking. DON'T leave two balls of play-doh with the leads connected touching each other for too long - this is a short circuit and will overheat the battery. Also, DON'T touch the legs of the LED directly to the alligator clips or the terminals of the battery. LEDs use 3V to operate, our battery puts out 9V. If you do this, the LED will burn out, and could potentially explode when heated making some plastic shrapnel.

Now that we have the safety talk out of the way, let's learn how to "plug in" the LED properly to our circuit. LED stands for Light Emitting Diode. A diode is an electrical component that only allows electricity to flow in one direction through it. When electricity is flowing the proper direction through an LED, it lights up! When it is flowing the opposite direction, nothing happens. So, how do you tell? LEDs have two legs on them - the longer leg is the positive lead, the shorter leg is the negative. So, make sure that the long leg of the LED is connected to the positive ball of play-doh, and the shorter is connected to the negative. If you plug in your LED and nothing happens or you can't tell which leg is longer, simply turn it around to see if this is the problem.

The picture above shows the LED working and properly connected for this test circuit. Congratulations!

Step 4: How Does This Work?

When I present this activity to kids in the classroom or in an informal setting, I usually ask whether they think play-doh will conduct electricity before plugging the LED in. This is a good way to engage the students about the class of materials we call electrical conductors, and their counterpart, insulators. Conductors of electricity are materials that electricity will readily flow through - this includes things like metal and water that has electrolytes in it. There are more exotic things like graphene sheets and such, but these two classes cover a large part of what kids will be familiar with when it comes to conductors.

Insulators are materials that electricity does not like to flow through. There are many more different materials that are insulators than conductors, so challenge the kids to come up with the goofiest insulator. Wood, paper, rubber, plastic, fabric, glass, etc. are all insulators.

Now, how does our LED that likes 3V not explode when connected to the play-doh, but will get damaged if hooked up directly to 9V? Well, the play-doh is conductive because of the water and salt in it, but it does not conduct ALL of the 9V to the LED. All conductors have a little bit of electrical resistance. This is just like it sounds, some of the current that flows through the LED is resisted by the play-doh - there are other ingredients in there like flour and oil and cream of tartar that are insulators, so play-doh has a relatively high resistance as compared to copper or just a cup of salt water. When the electricity flows through the play-doh, the resistance of the material brings the voltage down to 3V for the LED, and everyone is happy.

To dig a little further into this, push the two balls of play-doh together once your LED is lit up (it's okay to do this for a short time). The LED will go out, as shown above. I like to tell kids that electricity is very lazy - it will take the easiest way it can to make a completed circuit. When the two balls are separated, there is technically an insulator in between them - air. So, to complete the circuit the electrons have to flow all the way up through the LED, light it up, and go back down into the other ball of play-doh. When the two are pushed together, the electrons can flow easier through the play-doh vs the LED so do that instead. They take a shorter path to complete the circuit, which is why it's called a short circuit!

If you put a piece of paper, wood, foam, etc in between the balls of play-doh and push them togetner, your LEDs will still light as shown in the second photo above. Try adding more LEDs to the circuit - what happens to the brightness of each LED as you do this?

Lastly, most items that are insulators are really just materials that have a high resistance to electricity - electricity can still travel through them, but it takes a lot more current to overcome the high resistance of the material. This means that electricity can travel through air (think of lightning and static electricity) or melt through the insulation on a wire, it just has to have a lot of current.

Step 5: Make More Complicated Circuits

Now that we've got the very basic principles down for this, it's easy to expand to make more complicated circuits or experiment with other components. Try the piezo buzzer - what happens if you plug it in with reverse polarity? What about a motor?

The pipe cleaners are another great additional tool for this activity - they are essentially an insulated wire with fluffy plastic on the outside and flexible steel on the inside. You can use pipe cleaners to jump between balls of play-doh. If it's not working, strip some of the fluff off of the ends of the pipe cleaner. Remember, though, that you always have to have a ball of play-doh to plug into or you'll be putting 9V into the LED.

The best part of this activity is making your own creature - I've seen everything from snakes and ponies to castles to hamburgers with red LED ketchup!

Step 6: Explain, Expand and Evaluate

Explain and Expand:

What is a circuit, and what does it mean for a circuit to be open or closed?
What happens when you add more components to the circuit ? Why?
Why do you think a battery will overheat when short circuited?
What things do you need to create the simplest circuit you can think of? What about a more complex circuit?
What other materials like play-doh do you think would work for this activity? Bananas? Pickles?
As these circuits are left connected for a bit, the negative terminal tends to get a black film on it. Why do you think this happens (hint: it has to do with the salt content and electrolysis)?

Evaluate:

How many balls of play-doh do you think you could hook together before the resistance would be too great to complete the circuit?
How is it that static electricity can travel through the air (shocking your sibling or friend right before you touch them directly, for example) when air is a resistor?
What is resistance in a circuit? See above image for a graphical illustration of Ohm's law as it pertains to volts, amps, and ohms.