Kids like demos

Sometimes I get asked to do some demos for kids. I like this, it is fun. This week, I was asked to do just such a demo. Kind of as a historical record, I am going to document what I did.

The audience

It is always important to think about who you are talking to. Is this a group of high school physics students? High school teachers? Middle school teachers? Kids? It could be anything. In this case, I was meeting with a group of kids from ages 8 to 12 as part of a science camp.

The Objective

Is there a reason for the demo show? In this case, there was no stated objective. That means I can really do whatever I want. Sometimes, I try to have a theme or cover some particular topics. Other times, the show is mostly entertainment (but that has value too, I think).

Other issues

I like stuff that is not too heavy to carry. Who knows what help I will have when I get there. I hate making multiple trips to my car. If I can just carry all of it at once, that is perfect (I accomplished that in this case). Another consideration is setup. Suppose you want to do the "shoot the banana from the tree" (or monkey). This requires a bit of on-site set up. I would just rather not do that.

One final consideration. I like to use more homemade stuff than not. Oh, sometimes you just need some liquid nitrogen or something. But if I can do a demo with something the kids could go home and make - that is alpha super awesome bonus.

Science Tube

This "mystery tube" is one of my favorites for talking about the modeling nature of science. Basically, it is a closed tube with 4 strings coming out of it.


I explain to the kids that science is about making observations and making models. First, observe me pulling on this string. I then pull on a string so that it looks like one goes straight through (pull on the short end opposite the long one sticking out). What could the inside of this tube look like to make it behave this way? Most kids would say that there is a string going from one side to the other and the other two strings are just really short.

Ok, that is our model of the inside of this tube that we made based on our observations. What would our model say would happen if I pull one of the other two strings? Key points to emphasize: this is not "what do you think will happen" - this is what does the model say will happen. There are more than one possible model that they could have that would agree with the data.

I then discuss some possible models and pull the other string to show what happens. They usually get excited and say it is a magic trick or something. It is not magic.

Another important point. I actually did not make this tube (someone else made it). So, technically, I do not know what is on the inside of the tube. This is just like science. We can make models, but we never know the truth. We just have models that work and models that don't work.

Here is a video of this tube - oh, and look at it quick while youtube still has the vuvuzela button added.

Cool Serving Tray

This is one of my favorite demos:

And here is a basic intro to the physics.

For this demo, I go with the story that I have practiced serving drinks - because this is difficult. I then run around with it all over the room. Kids typically think the cups are glued down - so I let them take one off. The physics I include with this is that this is an example of strings. Strings can only pull. I know that is a stretch, but maybe the real answer is too complicated.

Balancing a stick with my finger

This is another fun demo - fun because you can do it with lots of different stuff. Here is a quick video:

Balancing a Stick Demo from Rhett Allain on Vimeo.

Again, I present this as a story of me being very skilled at something. Actually, there is just a little skill involved. Here is some physics of balancing sticks.

The main point I try to get across to the students is that things do not like to change their rotational motion. The more mass is farther from the point of rotation, the less it likes to change. This is a lot like forces and mass. I like to give the example of pushing and pulling a small boat versus a huge boat.

Friction Meter Stick

I didn't really have a story for this demo. Instead, I asked for predictions. If I move both of my hands inward, which will get to the center first?

Meterstick friction demo from Rhett Allain on Vimeo.

The only idea I talk about after this is that friction depends on how hard two surfaces push together. The hand that is closer to the center of mass has more friction. I know it is a stretch, but again - they like this one.

Moment of Inertia sticks

I don't say moment of inertia. Instead, I build two sticks with the same mass. Interestingly, if you ask which has the greater mass not all students will say they are the same. Anyway, I get two student to come up and play a game. The game is: see who can rotate the stick back and forth 10 times the quickest. I then have the students switch sticks.

Demo for Moment of Inertia from Rhett Allain on Vimeo.

The lesson here: rotational resistance to motion doesn't JUST depend on mass. It also depends on how far that mass is from the point of rotation. (oh - here is my post for this demo)

Cartesian Diver

This one is full of win. First, the kids can easily build something like this themselves. Second, I can bring two of these and pass them around.

Lesson: things float based on how much water they displace. When you squeeze the bottle, the air space gets smaller and the diver sinks. Cartesian diver post.

What is the color black?

I start with a question: what would happen if you were in a room with absolutely no light? Would you see anything? What if you waited a while? What color would you see? The answers to this questions were fairly evenly split with some saying you would see something and the others saying you wouldn't see anything.

To answer this question, I show them a box I made (just a box that looks black on the outside with a tube sticking out).


I go around and let everyone look inside. The question (that they should only answer to themselves) is: what color is the inside? After that, I go around and let them look again. This time, I open the door on the top. If I do this with kids or college student, about 90% laugh. If you want to see why, check out my description of this box.

Infrared Thermometer

This is just a cool toy. I briefly talk about stuff giving off light. Think of the element in an oven. When it gets really hot, it glows. Well, it still glows when it is cooler, but you just can't see it. This is infra red light. The nice thing about the IR thermometer is that I can pass it around and the kids can point it at stuff.


This took about 50 minutes to do this. I had more stuff, but you have to know when to stop. If you have a great demo you like, please share it in the comments. I will try to keep some more resources for others if you want to do some demos.

More like this

Along the same lines as the moment of inertia sticks, a similar concept can be learned using a fishing pole. I was fishing with a few split shot sinkers about 3 feet from my hook. Later I decided I wanted the hook closer to the lake bottom, so I moved half of the weights (because I was too lazy to move all) to about 1 foot from the hook. When I cast out, the line only went about half of the distance because energy was being used to fling the sinkers around each other.