Pumpkin Stacking STEM Challenge: Explore Balance & Center of Mass

How many pumpkins can you stack before the whole thing comes crashing down?

That question turned into one of our favorite fall engineering challenges.

In this pumpkin stacking STEM challenge, kids experiment with balance, center of mass, stability, measurement, and problem-solving while trying to build the tallest pumpkin tower they can.

The activity works for toddlers through older kids because everyone can explore it at their own level. Younger kids can stack, count, and compare pumpkins, while older kids can investigate why some arrangements are more stable than others.

Looking for more seasonal learning? Find this activity and dozens more in our 31+ Halloween STEM Activities for Kids.

The Science Behind Pumpkin Stacking

A stack stays upright when its combined center of mass remains positioned over its base of support.

Every time you add another pumpkin, you change the mass distribution of the entire tower.

If too much of the tower’s mass shifts outside the area supported by the pumpkins underneath it, gravity wins and the stack tips over.

Pumpkins make this especially interesting because they are not perfect blocks.

They have:

• Curved surfaces
• Ridges
• Uneven bottoms
• Different sizes
• Different stem shapes
• Slightly different centers of mass

That means stacking pumpkins becomes a real engineering problem instead of a simple block-building activity.

What Is Center of Mass?

The center of mass is the point where we can think of an object’s mass as being concentrated.

For a regular object with evenly distributed mass, that point may be close to the geometric center.

Pumpkins are not perfectly regular.

Some are wider on one side, flatter on the bottom, heavier near the base, or tilted by an uneven stem.

That means kids often have to rotate the pumpkin and test several positions before they find one that balances.

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Pumpkin Stacking STEM Challenge Supplies

You’ll need:

• 4–6 mini pumpkins
• Apples, optional
• Connecting cubes or another unit for measuring height
• Flat surface for building
• Ruler or measuring tape, optional
• Paper and pencil for recording results

We used 2 cm connecting cubes to measure the height of our stacks, which made the math very visual for younger kids.

How to Do the Pumpkin Stacking Challenge

1. Explore the Pumpkins First

Before stacking anything, give kids time to examine the pumpkins.

Ask:

• Which pumpkin has the flattest bottom?
• Which is the widest?
• Which feels heaviest?
• Which has the tallest stem?
• Which do you think should go on the bottom?

Have kids make predictions before they start building.

This turns random stacking into an actual engineering investigation.

2. Start with Two Pumpkins

Place one pumpkin on the table and try balancing a second pumpkin on top.

Rotate the top pumpkin slowly until you find a stable position.

If it falls, try again.

Ask:

What changed when you turned the pumpkin?

Why do you think one position worked better than another?

3. Add a Third Pumpkin

Once two pumpkins are stable, add another.

Now the entire tower’s center of mass changes again.

Kids may discover that the best bottom pumpkin is not necessarily the biggest one.

A slightly smaller pumpkin with a flatter surface may provide a more stable base.

4. Keep Building Until the Tower Falls

Continue adding pumpkins one at a time.

Each time, let kids:

1. Predict whether the tower will stay upright.
2. Choose where to place the pumpkin.
3. Test the arrangement.
4. Observe what happens.
5. Make a change and try again.

That cycle of design → test → observe → redesign is the engineering design process in action.

How Many Pumpkins Can You Stack?

Once kids have a successful tower, count the pumpkins.

Then challenge them to add one more.

Can they build a tower of:

• 2 pumpkins?
• 3 pumpkins?
• 4 pumpkins?
• 5 pumpkins?
• More?

Our kids managed to stack five mini pumpkins, although the shape and size of the pumpkins make every attempt different.

The goal is not necessarily to build the tallest tower immediately.

The useful part is figuring out why one design works better than another.

Add Math to the Pumpkin Balance Challenge

Once you have a stable pumpkin tower, measure it.

We used connecting cubes as nonstandard units of measurement.

Our original results were:

• 4 pumpkins = 10 connecting cubes = 20 cm
• 5 pumpkins = 14 connecting cubes = 28 cm

That gives kids several math questions to explore.

Compare the Heights

How much taller did the stack become when we added one pumpkin?

14 cubes − 10 cubes = 4 cubes

In centimeters: 28 cm − 20 cm = 8 cm

Estimate Before Measuring

Before adding another pumpkin, ask kids to predict the new height.

Then measure it.

How close was the estimate?

Compare Pumpkin Sizes

Measure each pumpkin individually.

Does the tallest pumpkin make the tower taller when it is placed on top?

Does placement change the final height?

Why Does the Bottom Pumpkin Matter?

The bottom pumpkin creates the base of support for the entire tower.

A wider, flatter base usually gives the tower more room to keep its combined center of mass over the supported area.

Try building the same tower with:

• The widest pumpkin on the bottom
• The smallest pumpkin on the bottom
• The roundest pumpkin on the bottom

Which arrangement is most stable?

This is an easy way to show that engineering is not simply about using the biggest object first.

Shape matters too.

Does Pumpkin Position Matter?

Absolutely.

Take one pumpkin and rotate it.

Try stacking it:

• Stem up
• Stem down
• Tilted slightly
• Resting on a flatter ridge

Does the tower behave differently?

Even though the pumpkin’s mass has not changed, the contact points between the pumpkins have.

That changes how stable the tower is.

Turn Pumpkin Stacking into an Engineering Challenge

Once kids understand the basic activity, add constraints.

Try:

• Build the tallest tower possible.
• Stack five pumpkins without using the largest pumpkin on the bottom.
• Build a tower using only three pumpkins.
• Create two different four-pumpkin towers.
• Build the most stable tower instead of the tallest.
• Make a tower that survives a gentle table tap.
• Use exactly the same pumpkins in a different order.

Constraints make engineering challenges more interesting because kids have to find new solutions instead of repeating what already worked.

Try an Apple Stacking Challenge

Once we finished with the pumpkins, we tried apples.

And apples were much harder.

Apples tend to have rounded sides and uneven bottoms, which give kids fewer stable contact points.

Our apples had also been used in other balancing and coding activities, so they were not exactly pristine by this point.

Try stacking:

• Two apples
• Three apples
• Apples of different sizes
• An apple and a pumpkin together

Then compare the results.

Which fruit is easier to stack? Why?

Compare Apples and Pumpkins

This makes a great extension because kids can compare two objects with very different shapes.

Ask:

• Which has a flatter base?
• Which has deeper ridges?
• Which rolls more easily?
• Which gives you more possible balancing positions?
• Which produces the taller stable stack?

Kids can use what they observe to explain why one material is easier to build with than another.

Add Art with Silly Pumpkin Faces

Once the engineering challenge is over, the pumpkins do not need to be finished.

We turned ours into silly pumpkin faces using our printable pumpkin face pieces.

That gives younger kids another way to explore:

• Facial features
• Emotions
• Symmetry
• Creative design

Try the Silly Pumpkin Faces Activity →

Pumpkin Stacking for Toddlers and Preschoolers

Younger kids do not need a formal explanation of center of mass to learn from this activity.

Let them:

• Hold the pumpkins
• Compare sizes
• Stack two objects
• Count pumpkins
• Feel the ridges
• Compare heavy and light
• Knock the tower down and rebuild it

That hands-on exploration develops early problem-solving, fine-motor control, spatial reasoning, and cause-and-effect thinking.

For preschoolers, ask simple questions like:

Which pumpkin should go first?

Why did it fall?

Can you turn it another way?

Pumpkin Stacking for Older Kids

Older kids can make the investigation more quantitative.

Have them record:

• Number of pumpkins
• Order of pumpkins
• Tower height
• Width of the base
• Number of successful attempts
• Time the tower stays standing

Then compare different designs.

Which arrangement was tallest?

Which was most stable?

Were those the same tower?

That distinction creates a great engineering discussion because the tallest design is not always the best design.

Questions to Ask During the Pumpkin STEM Challenge

Try asking:

• Which pumpkin should go on the bottom?
• What makes this stack stable?
• Why did the tower fall in that direction?
• What happens if you rotate the pumpkin?
• Does the largest pumpkin always make the best base?
• Can you build the same height with fewer pumpkins?
• Which matters more: size or shape?
• How could you make the tower harder to knock over?

Instead of telling kids what to change, let them use the tower itself as evidence.

What Kids Learn from Pumpkin Stacking

This simple fall activity connects several areas of STEM.

Kids explore physics through gravity, balance, and center of mass.

They use engineering when they test different arrangements and redesign unstable towers.

They practice math through counting, measurement, comparison, subtraction, and estimation.

And younger kids build fine-motor control and spatial awareness every time they carefully position another pumpkin.

Read a Book with Your Pumpkin Balance Activity

We love incorporating books into our activities. Here are some great books about science and pumpkins to read with your activity!

• Stack the Cats by Susie Ghahremani
• Mirette on the High Wire by Emily Arnold McCully
• Balance the Birds by Susie Ghahremani

Looking for More Easy STEM Activities?

If your kids enjoy simple challenges using materials you already have, explore our 31 Days of Low-Prep STEM and STEAM Activities for Kids.

You’ll find hands-on science, technology, engineering, art, and math activities that are easy to set up and designed to get kids experimenting.

Try More Halloween STEM Activities

Keep exploring balance with our Room on the Broom Balance STEM Challenge or build a Stellaluna Balance Bat.

You can also investigate pumpkins with our 5 Senses Pumpkin Science Activity, Mini Erupting Pumpkins, and more in our collection of 31+ Halloween STEM Activities for Kids.

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Explore the 5 Senses with a Pumpkin

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Pumpkins make great decorations. They also make surprisingly good engineering materials.