Pizza Triangles

Pizza Triangles

If you add up all three angles in a triangle, the angles at the tips, you will get 180 degrees.

Pizza Slices from Sofia's
Pizza Slices from Sofia’s (Photo credit: hicharice)

Have you ever heard someone say, “You did a complete 180 on that!” They mean that you turned around in the exact opposite direction. Imagine you are standing on a straight line. You turn half-way around to face the opposite direction. This is 180 degrees. That’s where that expression comes from.

How can you show that when you add the three angles of a triangle you have a straight line, which is 180 degrees?


  • Pizza
  • Kitchen scissors, sharp knife or pizza cutter
  • Three toothpicks
  • Optional: Piece of paper, crayon


  • Cut the pizza into eight slices
  • Cut the rounded crust off one of the slices, to make a straight edge
  • Stick one toothpick in each of the three tips of the pizza slice with the straight edge
  • Cut large tips off the slice with the straight edge, giving you three smaller triangles
  • Slide the tips next to each other, lining them up so that the tip of one triangle is next to the long side of another triangle and the tip of the third triangle, using the toothpicks to show you where the tip of the triangle was
  • Optional: draw a triangle with the crayon on a piece of paper. Cut out the triangle. Put a dot on each of the three tips of the triangle. Tear off the tips of the triangle. Line up the tips so the point of one triangle is next to the long side of a second triangle tip, then next to the point of the third tip.

What Should Happen?

The three triangle pieces will line up in a straight line — tip, edge, tip. You have just shown that the angles of the three tips of a triangle, when added together, side by side, equal a straight line, or 180 degrees.

Why Is This Useful?

Now, if you know two of the three angles in any triangle, you can find out the third angle, by adding the two you know and subtracting the sum from 180 degrees to give you the third angle.

“Angles are measured in degrees. The number of degrees tells you how open the angle is.” The maximum angle is 360 degrees, the number of degrees in a circle. A straight line is half of that, 180 degrees.

Now, you can eat the pizza.


Thanks to for their description of angles.

Thanks to for their description of this activity with pape and scisso

Click to watch a YouTube video showing the paper tearing activity.


Carol Covin, Granny-Guru

Author, “Who Gets to Name Grandma? The Wisdom of Mothers and Grandmothers”

Filed in: education Tags:

Comments (7 )

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  1. Susan Oakes says:

    Your grandchild is certainly going to have an interesting time learning all the things you writ about Cheryl.

  2. upliftingfam says:

    This is a great math lesson. Don't forget you can teach circles, semi-circle as well. 😉

  3. JeriWB says:

    Hmmm, maybe I would have paid more attention in match if pizza had been involved 😉

  4. I never would have thought of this with pizza. Food math is fun.

  5. Jeannette Paladino says:

    Cheryl — this is probably a good way to teach children about math. But I'll just stick to eating my pizza the old-fashioned way: one large slice at a time!

  6. yearwoodcom says:

    I love this lesson…I actually understood it and it doesn't hurt that you can use pizza, pie or cake to make the point. 🙂 If only our schools were so clever.