Why towers tip over
Your child stacks one more block, the tower leans, and down it goes. Or the cardboard robot falls flat the moment it stands up. It feels random. It is not.
Science teachers Aaron Isabelle and Gilbert Zinn explain it in one idea. Every object, or group of objects stuck together, has a center of gravity: the point where all of its weight seems to be concentrated1.
Every object, or combination of objects, has a center of gravity.1
Here is the rule that explains the falls. You can balance an object by supporting it anywhere on a vertical line that passes through its center of gravity1. For a tower, the support is the base. Picture an invisible string hanging straight down from the center of gravity. If it lands inside the base, the tower stands. If it lands outside, the tower tips.
A tower does not fall because it is tall. It falls when its weight point drifts past the edge of its base.
The balance point is not always the middle
Kids usually guess that things balance in the middle. That works for a ruler, because its weight is spread evenly. Tape a big eraser to one end and the balance point slides toward the eraser. The weight point follows the weight.
Your child already knows this from the park. A picture vocabulary book for early school years labels the seesaw with three words: balance, push up and gravity2. Ask them what happens when a big kid and a small kid ride it, and where the big kid has to sit to make it work.
Things only balance at their middle.
A tall tower is always a wobbly tower.
Height alone is not the problem. A lean, a heavy top piece or a narrow base moves the weight point toward the edge of the base, and that is when it tips1.
Words to use while you build
STEM words are tricky for young kids because many of them name things we cannot see, like forces2. The same book notes that talking about words together exposes children to extra words as part of the conversation2. So say these out loud during the build, not as a lesson before it.
The pull that brings everything down toward the ground.
Say it: “Which way does gravity pull your tower?”The one spot where all of a thing's weight seems to be1.
Say it: “Where do you think the weight point of this box is?”When the pushes and pulls are even, so nothing tips.
Say it: “What would make this side balance that side?”The bottom part that holds a build up and touches the ground.
Say it: “How could we make the base wider?”A string with a weight on the end. Gravity pulls it straight down1.
Say it: “Is the string pointing straight down or sideways?”Find the balance point of anything
This is the classic way scientists find the center of gravity of a flat shape, scaled down for a kitchen table. In the original, students hang a cereal-box rectangle from three holes in turn and draw where a weighted string falls. The three lines cross at one point, and the card balances on a pin right there1.
The cereal-box balance finder
- Big side of a cereal box
- Hole punch or a pencil tip (adult uses it)
- Thumbtack and a corkboard, or a pencil you hold level
- String about as long as the card, with a washer or binder clip tied on
- Marker and scissors
- Cut a rectangle from the cereal box. An adult punches three small holes near different edges.
- Hang the card from hole 1 on the thumbtack or pencil so it swings freely. Hang the weighted string from the same spot.
- When the string stops moving, mark where it crosses the bottom edge. Draw a line from the hole to that mark.
- Repeat from hole 2 and hole 3. Ask first: where will this line go?
- Find where the three lines cross. Balance the card on a fingertip right there.
- Now cut a lumpy, uneven shape and do it again. The method still works1.
- Adult supervision: an adult handles the thumbtack and hole punch and keeps small washers away from children who still mouth objects.
Challenge: three surprising balancing acts
The book's team challenge asks kids to build three different balancing-act models from simple materials1. It also asks the key research question: what is the secret to getting an object, or a group of objects, to balance1? Here is a home version.
- A cardboard bird that balances on its beak. Tape a coin under each wing tip and move the coins until the bird stays up on your fingertip.
- A ruler or paint stirrer that balances off-center. Tape a heavy object to one end and find the new balance spot.
- Ages 8-10, with an adult: a fork and spoon wedged together, a toothpick between the middle tines, the toothpick tip resting on the rim of a sturdy cup1.
- Sketch each one and draw a dot where you think the center of gravity is.
- Say why it works using the words weight point and support.
Have patience! The balancing act may take a while to set up.1
That teacher note is the real lesson: the setup takes many small adjustments1. When your child wants to quit, tell them the authors expected that too.
Turn a falling tower into a science moment
Researchers who observed teachers running engineering projects saw a common gap. Some teachers let kids build and test without ever asking why a design worked or failed, which the authors call an exercise in tinkering3. The more effective teacher talked about the science after the first round of testing3.
- Rebuilding the tower for them
- "Oops, it fell. Try again."
- "Make it balance in the middle."
- Explaining center of gravity before they build
- "Show me where it started to lean."
- "Where is the weight point? Is it over the base?"
- "Where do you think it balances? Test it."
- Naming the idea after the first test, when they have seen it happen3
| What you see | What is happening | Try this |
|---|---|---|
| Tower leans, then falls | The weight point moved past the edge of the base1 | Widen the base, straighten the stack |
| Top-heavy build topples | Heavy pieces up high lift the weight point and let it drift | Put the heaviest pieces at the bottom |
| Cardboard figure falls forward | The arms or head stick out past the feet | Bigger feet, or a small weight low at the back |
| Bridge sags on one side | The load sits far from the support | Move the load closer to a support |
What to expect at each age
Engineering educators Rogers and Portsmore, cited in a teacher-training book, name building sturdy structures as an appropriate engineering concept for kindergarten, linked to the science of forces, and adds gearing, motion and prediction in first grade3. The original balancing activities were written for upper elementary and middle school1, so we scaled them down.
Can: Spot that a build is leaning, guess where something will balance, and fix a tower by trial and error.
Try: Fingertip balance hunts, the cardboard bird, and "widen the base" tower rebuilds. Use the words weight and balance.
Can: Use the plumb-line method on their own, predict where the lines will cross, and explain why a build tipped.
Try: All three balancing acts, the lumpy-shape test, and sketching the center of gravity before they build.
If you want a kit for this
Everything above works with a cereal box, but if your child likes building things that stand up, a cardboard kit gives them more shapes to balance.
Build it with / Ages 5-7STEM Cardboard Construction KitKid-safe tools turn any box into a machine.$44.99GoP Toys sells the kit mentioned above. None of the sources cited tested our products.