Triangle Lesson for Kids: Definition & Facts

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  • 0:00 What Is a Triangle?
  • 0:31 Definition of a Triangle
  • 1:21 Types of Triangles
  • 1:47 Angles
  • 2:40 Lesson Summary
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Lesson Transcript
Ashley Pettway

Ashley is a Special Education teacher who loves inquiry-based, hands-on learning!

Expert Contributor
Kathryn Boddie

Kathryn earned her Ph.D. in Mathematics from UW-Milwaukee in 2019. She has over 10 years of teaching experience at high school and university level.

Triangles may be a simple shape but there are lots of different types! In this lesson, we will clearly define a triangle and think about some of the different kinds of triangles.

What Is a Triangle?

Triangles are very common shapes. We see them on road signs, on houses, and in food (think chips or pizza). When you think of a triangle, you might imagine three equal sides because we see this type of triangle a lot in our daily lives, but did you know that there are lots of different types of triangles? Triangles can be stretched out and tall like the Eiffel Tower or short and squished like a hanger. What are some other examples of different types of triangles that you can think of?

Definition of a Triangle

All types of triangles can be grouped or categorized. Triangles are categorized by the length of their sides and the size of their angles. Before we get into the different types of triangles, take a minute and think about what a triangle is. (Hint: 'tri' means three)

Simply put a triangle is a shape with three sides and three angles. With a definition that simple, there are bound to be lots of different kinds. As we explore all the different types of triangles, remember the three must-haves for a shape to be a triangle. The shape must have:

  1. 3 sides
  2. 3 angles
  3. All of the angles add up to 180 degrees

Types of Triangles

Like we talked about before, triangles are categorized by the length of their sides and the size of their angles. First let's look at the three different categories of triangles based on the length of their sides.

  • Equilateral: All sides are equal. All angles are equal.
  • Isosceles: Two sides are equal. Two angles are equal.
  • Scalene: No sides are equal. No angles are equal.

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Additional Activities

Activity - A Triangle Hunt

There are triangles everywhere in the real world. In this activity, we will hunt for triangles in real life and classify the triangles based on their side lengths (and angle measurements, too, for an extra challenge!)


  • Paper and pencil for writing down the triangle measurements and classification
  • Ruler or measuring tape
  • Protractor (if you wish to take on the added challenge of classifying based on angle measurements)


1) Look around for a triangle to measure. You can look inside or outside (with permission of course!) Some places you may find triangles are: different foods, street signs, hangers, parts of buildings, and more.

2) When you find a triangle, write down a description of the triangle (what the object is, where you found it, ...)

3) Take your ruler or measuring tape and measure all three sides of the triangle. Write down the three measurements.

4) Classify the triangle by its side measurements. Is it equilateral (all sides equal)? Isosceles (2 sides equal)? Scalene (no sides equal)?

5) Optional - Use a protractor to measure the three angles of the triangle. Write down the three measurements.

6) Optional - Classify the triangle by its angle measurements. Is it acute (all angles smaller than 90°)? Obtuse (one angle larger than 90°)? Right (one angle equal to 90°)?

7) Repeat until you have found and classified many triangles.

Through this triangle hunt, you will gain measurement skills and understand how to classify triangles. You may be surprised how many triangles are out there!

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