Building chain reactions is a fun parent and child project. (For ages 3 – 5, mostly this means the parent doing the design with the kids cheering you on. 6 to 9 year olds are more of an equal partner in the play. Kids age 10 and up who are into these sorts of engineering challenges will often take off with this project… after being taught the basics, they may keep building and building.)

Chain reaction set-ups are often called Rube Goldberg devices. These are named after artist Rube Goldberg who drew fantastical illustrations of overly complicated machines that do a simple task.

Introducing Kids to Chain Reactions

A key element of Rube Goldbergs is that there is a series of chain reactions. Action A triggers action B, which then causes action C, and so on, until the goal is accomplished. There are lots of YouTube videos of AMAZING, complex Rube Goldberg devices. (Click on that link for a playlist of my favorites. There’s apparently also a ton of #rubegoldberg posts on Tiktok, but I haven’t gone down that rabbit hole yet.)

But if you show those elaborate videos to a child and then say “let’s build one of those”, they’ll end up super frustrated and disappointed in themselves because there’s no way they will accomplish something anywhere near that cool. Most of those chain reactions were set up over the course of many days, by people with years of experience playing around with these ideas, and some of these videos “cheat” and use camera angles or filming edits to make it look more impressive than the actual device is. So, I also collected a playlist of simple, achievable Rube Goldberg devices that can set more realistic (though still challenging!) goals to aspire to. You could show some of these simple videos, and then say “do you think we could make a chain reaction?” Then start the experimenting.

Set an achievable goal

Start small – you can always get bigger later if you’re having fun! Try for a 3 part process:

• Trigger a reaction to start a process that accomplishes a goal

For example, we challenge our students to build a pyramid of three toilet paper tubes and find a complicated way to knock it over. Pushing a toy car so it bumps into a ball is a trigger that starts the process of the ball rolling toward the goal of knocking down the tower.

Even with this basic 3 part process, the chain reaction usually won’t work perfectly on the first try. You’ll usually have to do a lot of tinkering with the design.

Chain reactions are actually one of the best ways to explore the engineering process: deciding what challenge you want to take on, looking at available supplies and brainstorming options, starting to build and tweaking as you go along to get to your goal. There will also be times where you’ve worked and worked to set something up, and then accidentally trigger it and have to start all over again! It can be super frustrating, so it’s a good chance to practice emotional regulation and build grit. Celebrate the failures as part of the process. (If you watch the videos on YouTube, many will show their “bloopers” and many say things like “attempt #47” to make clear that you should expect that!)

Once you’ve got a three step chain reaction working, then try adding on another process in the middle for a 4 step device, and so on and so on. You can think of each element as part of a modular system that can be swapped in and out, or arranged in a different order, to accomplish different goals. Test each segment several times to make sure it’s reliable before you try combining it with other elements.

Supplies

It can be hard to predict exactly what you’ll need. Often when building a step you just have to brainstorm in the moment, then run around the house or classroom to find just the right thing. But here are some ideas that might get you started. We do this exercise after studying simple machines, so I’ll reference those below.

• Wheels and other things that roll – balls, marbles, beads, dowels, cans, oatmeal tub, toy cars, batteries, skateboards, roll of tape
• Levers and Things that tip over – dominoes, blocks, cereal boxes, books
• Inclined planes (ramps) and tracks – boards, books, PVC pipes, tubes, cardboard, marble runs, gutters, car tracks or toy train tracks
• Things to build structures with – building toys (Lego, wooden blocks, K’nex, etc.), cardboard
• Things to fill up or dump – containers, buckets, cups, funnels and weights to fill them with (coins, blocks, water)
• Pulleys, gears, walking wind-up toys, action figures or miscellaneous toys, craft sticks
• Fasteners – tape, string, rubber bands (tip: if you tape down the items in the device that you don’t want to move, they’ll stay where you want them, and you won’t have to keep setting them back up over and over again)
• Pencil and paper? Many people recommend that you draw out your device before building… I personally tend to jump straight into the build, but do what works best for you!

Location considerations: First, work somewhere with plenty of open space; that’s not someplace that family members need to walk through or use several times during the period you’re working on the device. Second, gravity is the easiest way to power the actions of a Rube Goldberg, so typically the action starts on a higher surface and moves to a lower surface… so, something hanging up high knocks down something that’s set on top of a box that’s on a table, then it drops down to the level of the table, then off the edge of the table to the stool, then down to the ground.

Let’s look at some big picture ideas of how chain reactions are built, to hopefully inspire you to think broadly about your options. Then at the end, I’ll give some sample “assignments” that are easy to accomplish. Since I recommend starting at your final goal and working your way backward, that’s how I’ll organize the components below.

Goals

The final step of your device is to complete a task of some sort. Some examples are: turn on/off a light, fill a glass with water, water a plant, drop a bottle in a recycling bin, pop a balloon, break an egg, ring a bell, put out a candle, close a door, turn a page, raise a flag, land a ball in a basket, or knock over a tower. Decide what you want to try, and then figure out what you would need to do to accomplish that. For example, if your goal is to put out a candle, maybe the step before that is to dump water on it, so the step before that is to figure out how to knock over a cup of water. Or if the goal is to ring a bell, first you figure out how to set up the bell, then figure out what kinds of things could bump into the bell to ring it, then figure out how you’re going to move those things to the bell. Here are some optional actions to get you to your goal:

Components of a Chain Reaction

Domino Chain

This is the most classic element of a chain reaction. You can build it with dominos, or wooden blocks, or books, or mac and cheese boxes, or anything that’s in that flat rectangle shape that will stand balanced up on one end until tapped, and then fall over. For a child, the first challenge is learning what spacing is needed between blocks. If they’re too close, they may not hit with enough impact to knock the next one over. If they’re too far away, they hit low on the center of gravity of the next block and instead of tipping over, it just bumps out of the way. Once you’ve learned to set a chain of 3 blocks up, it’s easy (though tedious) to set up a chain with another 100 or so blocks. The hard part is not accidentally knocking them over too soon! (If you don’t want to accidentally trigger it too soon, you could put in a barricade here and there to make sure that IF a chain reaction started, it couldn’t get too far – or leave some gaps in a domino chain, and fill in those gaps at the last minute. One expert even recommends that once you’ve figured out placement you tape one side of the domino in place, so it acts like a hinge. When the domino gets knocked down, the tape holds it in the right place so you can just tip it up again.

Rolling Down an Inclined Plane

You can roll a ball (or a can, bottle… anything round) or anything with wheels down a ramp to trigger the next stage. (On a steep enough incline, you can slide down ANY low friction item.) Many Rubes start with this, but it can also be used in the middle of a device… you just have to have a way to stop the ball from rolling until it is time – there could be a flat platform at the top that it gets bumped off of, or there could be a gate that gets opened up to release the ball.

Ramps are usually just flat objects (boards, books, etc.) propped up on something (like a block or a book.)

Some devices have elements where an item rolls UP a hill. You have to get the weight and impact speed just right to have one item bump the next item up the hill.

Switchback Ramps

Once you’ve got one ramp working, it’s often possible to add more ramps in a back and forth pattern, and this is a fun way to build the drama.

Roll Down to Pull Up

If you’ve got a car rolling down a ramp, you can tie a string to it, and then tie an object onto the other end of that string. When the car rolls down, it pulls the other object up.

Zipline

A zipline is really just a form of inclined plane. Something up high slides down the line.

Tunnels

It’s fun to cover part of a ramp over. If the moving object goes into a tunnel, that’s exciting as it disappears for a moment, and you hold your breath as you wait for it to reappear and continue on its path.

Screws to Move Items Up or Down

This can be simple – just using a spiral ramp to roll a ball down – it’s fun to watch the ball go around and around. Many marble maze toys include a spiral ramp, and some toy car race tracks have spiral ramps or you can build one with paper plates and a poster tube. Or you can use a funnel to get a gravity well effect, which is basically the shape of the screw. Another common use is to put a ball (or other item) on a string, and wind it up around a pole. Then when you release it, it spins around and around, with the string getting longer till the ball hits its target. (Here’s a video that includes the funnel and the tetherball effects.)

You can also use a screw to lift objects upward like an Archimedes screw. This takes more fine-tuning.

Levers

The most common lever we see is a first-class lever, seesaw style. Something falls onto the end of the lever that’s up in the air, pushing it down, and launching the other end of the lever up. Catapults are one form of lever you could use to launch an item.

Pendulum

A heavy item on a string or rope swings from a fulcrum to knock into something.

Pulleys and Weights

A classic use for this machine is to have a pulley with baskets / buckets tied on each end of the rope. When something drops into the basket that’s up high, it goes down, bringing the other basket up. This mechanism is often used with a water drip… more and more water is added to a bucket till it gets heavy enough to go down.

You can also just simply tie a string onto any object, then add some weight to the other end of the string. When you knock the weight off the table, it pulls on the string and drags the object across the table.

Bounce

Yon can either use a bouncy item, like a rubber ball, or create a bouncy surface, like a trampoline. When something drops down, it bounces back up. If you get the angles just right, you get both that vertical change and a horizontal change from one part of the room to another. One way to make a bouncy trampoline is to cut the “neck” off a balloon, then stretch the remaining part of the balloon across the top of a cup or other container.

Triggers / Releases

Setting up a chain reaction takes a lot of work and a lot of time. It all has to be poised on the verge of the chain reaction, but then wait in that state until the exact moment you want the reaction triggered. So, there are a variety of ways to hold elements in place till it’s time for the reaction to start. You could lift a gate, swing a pendulum, tap on a domino. Some devices start with a cell phone resting on an inclined plane. When you call the phone, it vibrates, and the vibration causes it to slide down the ramp and trigger the chain reaction. Some start with a pet nudging something, or walking away and having their leash tug on a trigger. Some start with a fan blowing, a mousetrap snapping, a candle burning through a string, or a wind-up toy walking into something.

You have to have just the right amount of force to trigger the next item in the chain. One of the easiest ways to adjust force is with gravity – like if a ball rolling down a ramp hits too hard, make the ramp less steep. If it needs to hit harder, make the ramp steeper.

Some Set-Ups to Try

I’ve grabbed screen captures of lots of elements from videos in my playlist of simple, achievable Rube Goldberg devices. These are all elements that use simple materials you have at home. You can click on any picture for a better look. Pick out a few to try. If you get a couple separate elements working and you’re having fun, you could try stringing multiple elements together.

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Toy Car on a Ramp to Ring a Bell or Pop a balloon. Set up a ramp for the car. Find a way to trigger the car to start it moving so it rolls down the ramp, hits the bell hard enough to ring it, or hits the balloon hard enough to pop it. Hint: if it’s not hitting with enough force – you may need to make the ramp steeper to make it roll faster. (For the balloon: Put a pushpin, tack or needle on the front of a toy car.)

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Knock down a domino chain. 1) Roll a marble through a ramp made from a paper towel tube, 2) include fidget spinners in a domino chain – they help you change direction, 3) use a ball to knock over book-dominos to knock into another ball, or 4) set up a domino chain to travel UP a stack of books.

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Levers

1. Mount a wooden bar so you can lift one end up, let it go, and it will swing into a water bottle and tip it over to fill a cup. (You could get the same effect with a pendulum mounted on a string.)
2. Set up a seesaw lever of a ruler balanced over a marker, set a ball on one end of the ruler, then knock over a book so it lands on the other end of the ruler and launches the ball in the air.
3. The cereal box takes some work, but is a fun challenge: Poke a pencil through an empty cereal box, then balance that on two Lego columns. Put just a little cereal inside. Play with how to get it to tip to pour out the cereal. Then tie a string to it to prevent it from tipping, and tuck that string under a block… now think about what chain reaction step you want to use to knock the block off the string and make the cereal box tip. (Photo and idea from Hands On As We Grow, and idea from here)
4. Set up a lever seesaw of a cardboard plank over a toilet paper tube. Put the car on one end, then use a string to pull up that end. The car drives to the other end of the ramp and pops the balloon.

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Pulleys

1. Set one pulley up high, and another down low. Tie one end of the string to a bucket of items, then bring the string up through the high pulley, down through the low pulley, and attach to a clothespin. What can you make land on the clothespin to make it open, which will let the bucket fall?
2. Set up a pulley so when the ball falls in the bucket, the other end of the rope will pull up a ramp to make the car roll.
3. Set up a pulley so when car #1 falls into the bucket and the bucket goes down, the pulley will lift up the lever platform that car #2 is on so car #2 will roll.
4. Tie a flag on a string. Put the string over the top of a cardboard “flagpole”, then tie a tennis ball on the other end of the string. When the ball is knocked off the table, it pulls down one end of the rope, which brings the flag up.

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Ramps / Inclined Planes. One of the easiest ways to move something from one place to another is to roll it down a hill. So, there’s LOTS of inclined planes in chain reactions. See the list of supplies at the top and these pictures for ideas of materials to use. Once you’ve figured out one ramp, it’s fun to set up switchbacks, where your ball (or car or…) rolls one direction, then the other and so on as it goes down. The most intriguing ramp for me is the bottom, where they cut paper to create a spiral ramp around a water bottle. The geometry of it is challenging!

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Bounce and Pour: Try playing with bounces – 1) the ball falls from the table, bounces off one pot and into another – you have to have your angles just right. 2) You can create a “trampoline” by stretching part of a balloon over the top of the container. A common goal of Rubes is to “water a plant” – basically, knock a cup of water over so it pours onto a plant. 3) You could roll or bounce a ball into the cup of water, or you could 4) knock over a series of domino-books to knock over the water.

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Triggers. To get the action started, you can 1&2) have something fall into a car to push it down a ramp, 3) roll a roll of duct tape into a domino chain, or 4) have a cell phone vibrate its way down a ramp to hit the domino chain.

That’s just some ideas to get you started. As you can see, the only limit on Rube Goldberg design is your imagination!

More about Rube Goldbergs

This article in the Verge talks about the history of Rube Goldberg, and about modern RG competitions.

This video provides a brief introduction to Rube Goldberg devices and the annual competition, but it also talks about how parents and children working together on devices provided a welcome distraction during the coronavirus lockdowns. They touch on the fact that building a chain reaction requires a great deal of patience, persistence, and the willingness to try again and again to get it right.

More resources

Here are the most helpful explanations I found for teaching kids how to build chain reactions.

• WikiHow: How to Build a Homemade RG
• Tinkerlab Build a Rube Goldberg
• Hands On As We Grow: RG to Pour Cereal
• Brain Power Family: Rube Goldberg Devices
• Connections Academy Build your own RG
• This Sesame Street video

Lesson plans for middle schoolers: https://www.teachengineering.org/activities/view/cub_simp_machines_lesson05_activity1 and https://www.teachengineering.org/lessons/view/cub_simp_machines_lesson05

Look here for more ideas for fun Contraptions activities for kids age 3 – 6. That post includes recommended children’s books that relate to contraptions or Rube Goldbergs.