What makes a triangle a triangle
In 1st grade, children learn which features make a shape what it is, like three straight sides for a triangle; and which do not matter, like color, size, or which way it is turned.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 1st grade practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
What must be true for a shape to be a triangle?
- It must have 3 straight sides.
- It must be blue.
- It must be big.
Why this matters
Learning to look past how a shape usually looks, to what actually makes it a triangle, is your child's first taste of careful reasoning in geometry. In 2nd grade they name shapes by their sides and corners, and in 3rd grade they sort shapes into families, such as seeing that a square is also a rectangle. Children who rely on looks alone often miss a triangle that is turned on its side.
Where kids can get stuck
- Rejecting a triangle that is upside down or long and thin, because it does not look like the usual pointing-up picture.
- Calling a square tilted on one corner a diamond, and therefore not a square, because it has been turned.
- Accepting a shape with a gap or a curved side as a triangle because it has three corners.
How you can help
- Hunt for triangles around the house or neighborhood, like yield signs, pizza slices, and roof shapes, and talk about why each one counts as a triangle even though they look different.
- Build shapes with straws, uncooked spaghetti, or craft sticks. Ask your child to make a triangle, then a different triangle, and explain what makes both of them triangles.
- Draw a few shapes, including some that are almost triangles, such as one with a curved side or a gap, and ask your child to say which are triangles and why.
Want a little more? Watch the kid-friendly video below together, or try Bobby's Math Club for free practice that adapts to your child.
Kid-friendly explanation
Questions parents ask
Why is my child learning about defining attributes of shapes?
Learning which features define a shape helps your child understand shapes instead of recognizing them by appearance. A triangle is a triangle because it is closed and has three straight sides, not because it points up. This reasoning supports sorting and describing shapes in later grades and builds careful thinking.
Is a square a rectangle?
Yes. A rectangle is a four-sided shape with four square corners. A square has four square corners too, so it is a special kind of rectangle with all sides the same length. Many 1st graders find this surprising. It comes up again in later grades when children sort shapes into groups.
What does non-defining attribute mean?
A non-defining attribute is a feature that does not change what a shape is. Color, size, thickness, and which way it is turned are examples. A blue triangle and a yellow triangle are both triangles. Helping your child notice what changes a shape and what does not is the heart of this skill.
Make learning math fun
Bobby's Math Club turns 1st grade math practice into something fun to do together.
- Unlimited practice, completely free
- Questions matched to your child’s level, a few minutes a day
- Level-ups, badges, and a cheer for every win
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.
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Join the clubNote for teachers: This skill is covered by Common Core math standard 1.G.A.1: “Distinguish between defining attributes (e.g., triangles are closed and three-sided) versus non-defining attributes (e.g., color, orientation, overall size); build and draw shapes to possess defining attributes.” Learn how Bobby's Math Club can help your students grow. (Common Core 1.G.1)