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Building Block Towers


Active Time
20-30 minutes
Total Project Time
20-30 minutes
Key Concepts
Length, area, ratio, stability, forces
Ben Finio, PhD, Science Buddies
Green blocks are stacked low and wide while red block are stacked narrow and high


If you have ever built a tower with blocks, you have probably knocked it over—either on purpose, or by accident. Sometimes your tower gets too tall and wobbly, and you just cannot keep it standing! In this activity, you will learn about the trade-offs you need to make when building a tower that is both tall and stable.

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


  • Toy cardboard or wooden blocks, or an assortment of empty cardboard boxes (cereal/tissue/shoe boxes, etc.). Do not use plastic snap-together blocks like LEGO® bricks.
  • Optional: chair to stand on, or an adult to help you put blocks at the top of your tower.
  • Pencil and paper

Prep Work

Make sure you have enough open floor space to build a tall tower, and that the tower will not break anything if it falls over.


  1. Gather all your blocks before you start. Count how many blocks you have, and sort them roughly by size (e.g. into small, medium, and large piles).
  2. Make sketches of a few different block tower designs. Try to make the towers as tall as possible, but also plan for them to be stable, so they will not fall over easily.
  3. Try building one of your tower designs. If the tower gets higher than you can reach, stand on a chair or ask an adult to help you add more blocks. Can you finish building the tower before it falls over?
  4. If you manage to finish building the tower, try gently tapping it with your finger in a few places. Does the tower fall over?
  5. If your tower fell over, think about how you can redesign it to be more stable. Do you need to sacrifice some height to accomplish this?
  6. If your tower did not fall over, push the limits! Can you make it even taller while keeping it stable?
  7. Keep iterating (redesigning and rebuilding) your tower until it is stable—you cannot make it any taller without making it unstable.

What Happened?

Building the tallest possible tower might seem very simple: just stack all of your blocks vertically end-to-end, one on top of the other. That should give you the maximum possible height, right? Technically, yes—but it will also result in a tower that is extremely unstable because it is too tall and skinny. It might fall over on its own before you are even done building! Conversely, you can build an extremely stable tower by making it very short and wide—but then it will not be very tall! The best solution probably lies somewhere in between, with a tower that is wider at the base than it is at the top. You probably had to go through multiple iterations of building your tower until you found the design that worked best. This is a normal part of the engineering design process.

Digging Deeper

What determines if a block tower will stay standing or fall over? Think about a single rectangular block placed horizontally on the ground. The block has a wide base relative to its height, so it is very stable. If you push on its side with your finger, it will probably slide sideways, but not tip over. Now take the same block and stand it vertically on one end. The base is now narrower relative to its height. If you push on the block near the top, it will probably fall over instead of sliding. What if you add more blocks? Two rectangular blocks stacked on top of each other horizontally will be much more stable than if you stack the same two blocks end-to-end vertically. The tower's stability depends on the ratio of its width to its height. This poses a challenge: given a limited number of blocks, how can you build a tower that is as tall as possible, but also stable (i.e. it will not tip over easily when you push it or blow on it)? That is what you did in this project!

Note that this same concept applies to real buildings. Very short structures, like single-story houses, are generally very stable. So are tall structures that get narrower as they reach the top, like pyramids. Tall, skinny structures like skyscrapers and bridges can be much more susceptible to natural disasters like high wind and earthquakes, which can cause them to fall over. It takes a lot of extra engineering to make sure they stay safe!

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For Further Exploration

  • Aim a fan at your tower to simulate wind. Does your tower stay standing?
  • Build your tower on a table, then gently shake the table to simulate an earthquake. What happens?

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