The Power of Triangles

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The SWBAT identify that the triangle is a strong and powerful shape used in building.

Big Idea

Who is the strongest of them all? The students will investigate both the square and the triangle to discover the answer to the question.

Teacher Notes

Activity Description

The children have learned some of the basics of building.  In the previous lesson, the children learned that wide bases add stability.  Now they get to take their knowledge and put it to the test by building their own squares and triangles to see which one is the strongest.

NGSS/Common Core Connections

This lesson is a prerequisite lesson for tower building.  The children need some background knowledge as to what shapes make strong design components.  The NGSS standard connection is discovering the structure and function of designed objects are related the their function.


click for photo of materials

  • 4 brads per student
  • file folders--I recycled old ones
  • 1 small plastic bag per student filled with the cut file folders (see instructions below)
Advanced prep:  
  • Cut file folders 5 ½ inches long and 1 inch wide--4 per partner group
  • cut file folder 8 inches long and 1 inch wide--1 per group with a hole punched only on one end (see note below)
  • Each group needs a set with these 5 pieces (4 short and one long) and 4 brads.  I put each of these 5 pieces in a small plastic bag (click for photo of sample bag).

Before you punch the holes, lay the four same sized piece together so they form a perfect square.  Center the paper punch in the middle of where the two pieces meet.  Now that you know where the hole needs to go, start punching the rest. If you have a parent helper who likes to do work at home, this is the project for them!

If you want a physical copy of these directions, click here.



5 minutes

This lesson is an elaboration of the previous lesson Mighty, Mighty Triangles.  In that lesson, the children learned which features of structures add stability and strength, such as wide bases and triangular shapes.  This is why I consider this part of the lesson an elaboration, not an exploration.

I start the lesson with a quick review of the previous lesson about the stability of a wide base.

 Yesterday we learned something about stable shapes.  Who can explain what we have learned? What about their bases?

I am trying to get the children to remember the idea that structures that have a wide base are more stable than ones without.  Also, we did talk about how triangles have this base, so they should be able to reflect on that idea as well.

Remember when we looked at example of towers we noticed that they had wide bases.  Some of them were square and some of them were triangular.  If the two shapes, a square and a triangle were to see which one was the strongest, who do you think would win?

Most kids vividly remember yesterday's lesson, and start placing their "bets" on the triangle. But others are not sure of this idea and want to experiment to figure it out.  The idea that triangles add strength in building cannot be emphasized enough, so let the betting begin!  


25 minutes

I give each child a bag with the 5 file folder strips and brads in it.  I give them a quick mini lesson on how to use the brads.  I only have them take out 4 of the strips to begin with.

Take out 4 of the strips that are the same size and all of the brads and lay them on your tabletop.  The LONGER strip should be left in the bag.  You are invited to make a square with the 4 same sized pieces.  Use the brads to secure the shape.  After you make the square, test it to see if it is strong.  Gently push on the sides to see how much pressure makes the shape give in.  See a square is not stable video clip.

As they are exploring I walk around and note their observations.  

Now I would like you to carefully take one piece out.  You will have to undo the brad and take the piece out.  Then reattach the pieces together.  What shape have you formed?  Test the triangle in the same way.  Gently push down on it and see what happens.  

Watch this video clip to see the excitement of two girls explaining both the strength of the triangle and the square.  I love how they use the word stable in their explanation.

When I think they have completed the task, I have them add the last piece to the others.
I would like you to take out the last piece.  It is a little bit longer than the rest.  How do you think you could add this piece in with the others?  Go on ahead and try it out.

If they need some prodding, I show them how to add the last piece in by making it go diagonally across the square shape.  When they have the longer piece in place, they should insert the brad.
What difference did it make to add the piece in?  Did it make your shape stronger?
Talk about your observations with your partner.  


10 minutes

To wrap the lesson up,  I hand out thPower of Triangles Observation paper.  

Record your observations on this recording sheet. To record, you should draw a sketch of the shape, label its parts and write your observations.

I want the children to make the connection that adding the last piece makes the shape stronger.  I was very pleased to see that some children made the connection that by adding the last piece, you are actually creating 2 triangles.  Their understanding has come so far!

When they are done, we glue the sheets in their science notebook