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Jessi: Have you ever wondered how a flashlight works? Sure, it seems pretty simple, all you have to do is move a switch or press a button, and... ta da! The light comes on! But what really makes the light go on? [squeaking] Right you are, Squeaks! It’s electricity that makes the light go on.

You probably already know something about the electricity that powers things in your house. Well, the electricity in a flashlight comes from the batteries that are inside. Electricity flows from the battery to a light bulb. And when the electricity goes through the bulb, the flashlight lights up! But the electricity doesn’t just jump from the battery right to the bulb. In order to get from the battery to the light bulb, the electricity has to go along a specific path. And that path is called a circuit.

If you listen to the word "circuit", it kind of sounds like a shape you might know. Let's name some shapes, and see which one sounds like “circuit!” Triangle... no... square... no... I know, circle! And if you look at a circle, you see that it goes around and around and doesn't have any spaces or gaps in it. And that’s true of a circuit, too! There can’t be any spaces in the path that electricity takes to get from a battery to a light bulb.

In a circuit, if electricity doesn't have a path to travel on, it stops. It’s a little like a train on train tracks... when there’s no more track, the train stops. So let’s see how a simple circuit works.

The battery is what we call the power source. That is, it’s where the electricity comes from in this circuit. Electricity flows from the power source on the path of the circuit. And a lot of the time, that path is made by some kind of a wire. The circuit then leads to whatever we want to power. In this circuit, that’s a light bulb. As the electricity goes through the bulb, the bulb lights up. The electricity leaves the bulb, and flows back through the battery. Then it follows the circuit all over again!

As long as there are no spaces in the circuit, the bulb will stay lit. But what if we want to turn the light on and off? Then we have to break the circuit, which means we need to put a space in it somewhere to stop the flow of electricity. So... we use a switch. A switch allows us to make a space in the circuit if we want to. When the switch is on, there’s no space in the circuit, and the light bulb is lit. But when we move the switch off, we open up a space. The electricity can’t jump over the space, so it can’t reach the light bulb, and the bulb stays off!

Okay, so I’m going to try to build my own circuit using some of the parts that I have here. I have a power source: this battery and a wire for the electricity to travel through... and a little light bulb. If I arrange everything just so, it kind of looks like a circle! But the bulb isn’t on. That’s because I haven’t connected all of the pieces together. Remember, there can’t be any spaces in the circuit.

So I’ll connect the wires to the power source... then I’ll connect one wire to the bulb... And when I connect the other wire to the bulb and complete the circuit... the bulb lights up! Now, I’m going to add something else to make it work more like a flashlight does. I have this part that looks like a button. And I’ll put it right here. And look! The bulb is lighting up. That means that there are no spaces in the circuit.

Okay Squeaks, Let's press the button and see what happens. Hey, the light bulb went off! That means when I press the button, the circuit is incomplete... which means that I've created a space. And when I let go of the button, the circuit is complete again. The electricity from the battery can’t cross the space, so it can’t get to the bulb and make it light up. The button is a switch, kind of like the switch on a flashlight!

If you look around, you’ll find that circuits just like this are everywhere, powering all of the things around you! Cars, trucks, and buses all use a circuit with a battery to get started. And circuits run through your refrigerator, too, and in all of the other machines in your home that use electric power in order to work.

Inside devices like phones and computers -- which you’re using right now -- there are very, very small circuits that use the same basic set-up as the one I just built! Large or small, it takes a circuit to carry the power we need to get going!

Thanks for joining us on SciShow Kids. And special thanks to Google Making Science for helping us make this video! Do you have a question about something? Grab a grownup and ask them to help you leave a comment on this video, or send an email to kids@SciShow.com! Thanks, and we'll see you next time, here at the fort!.