Triangles and Quadrilaterals

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1 Getting Started
2 Watch and Notice
3 Try It Together
4 Draw Them in Your Copy
5 Class Challenge
6 What Did We Notice?
7 What's Next
Pupil practice

1 - Getting Started ~4 mins

Illustration for Getting Started Look at these three triangles on the board. One is tall and thin, one is wide and flat, and one is very pointy. They look really different from each other.

Hands up: what is the same about all three of these shapes?

Teacher Notes

Take three hands-up answers, not open call-outs. You are fishing for they all have three sides — but accept and revoice anything close ("three corners", "three lines"). Do not name the rule yet; the model step does that.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~9 mins

The shape inspector lights up one side at a time. Count the sides with me as each shape appears. What is the same about the last two shapes?

Teacher Notes

Triangle: count together, three straight sides, three corners. Land that three straight sides makes a triangle whatever it looks like.

Square: four straight sides, four corners. Introduce quadrilateral here, say it slowly, class repeats once.

Rectangle: looks longer, but count again, four sides. Ask is this a square? (no) how many sides? (four). Four sides is the test, not how it looks, so a rectangle is a quadrilateral too.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~10 mins

We will look at four shapes together. For each one, count its straight sides aloud with the class, then decide: triangle, quadrilateral, or neither? Don't judge by how it looks, count the sides.

A tall thin triangle

A wide flat triangle

A stretched quadrilateral

A five-sided shape, neither one

Teacher Notes

Work the four shapes one at a time. Let a pupil reveal the sides, class counts aloud as they light up, then a quick verdict. Revoice each correct answer, e.g. three straight sides, so it is a triangle. Keep each shape brisk.

Tall thin triangle: 3 sides, triangle. Point that tall and thin doesn't change it.

Wide flat triangle: 3 sides, still a triangle. Looks nothing like the first one on purpose, count rather than guess.

Stretched quadrilateral: 4 sides, quadrilateral, even though it is long and stretched.

Five-sided shape: 5 sides, too many for either, so neither.

+10 XP

4 - Draw Them in Your Copy ~6 mins

COPYBOOK MOMENT

In your maths copy, draw two different triangles and two different quadrilaterals. Make them look really different from each other, the way the ones on the board did.

Under each pair, write the group name: triangles under your two three-sided shapes, and quadrilaterals under your two four-sided shapes.

Teacher Notes

Walk the room glancing for: two genuinely different shapes in each pair (not two near-identical squares), and the correct group name written underneath. No marking — this is whole-class copybook practice, not assessment. Give pupils enough time to draw four shapes carefully with a ruler.

+10 XP

5 - Class Challenge ~12 mins

Now we build shapes with straws at the visualiser. A few pupils take turns to come up and build each shape, while everyone else checks the build by counting the sides aloud. Show a thumbs up if you agree the shape is right.

Build a triangle.
Build a square.
Build a different quadrilateral that is not a square.
Try to build a triangle using four straws, and tell us why it cannot be done.

Hands-on Task

Teacher Notes

This is a single visualiser demonstration, not desk work: a few pupils take turns building at the visualiser while the watching class checks each build by counting the sides aloud and gives a thumbs up to agree. Straws are optional — if you have none, draw each shape on paper under the visualiser instead. Keep the board work brisk rather than over-explaining.

Watch for the make-or-break moment at the last task, and give it plenty of time: four straws cannot close into a triangle because a triangle needs exactly three sides. Let a pupil try and fail, then revoice: "a triangle has three straight sides — four straws makes a four-sided shape, a quadrilateral, not a triangle". For the "different quadrilateral" task, accept any closed four-sided shape (a long thin one, a kite-ish one) — the point is four sides, not a neat square.

+15 XP

6 - What Did We Notice? ~5 mins

MATHS TALK

One pupil says a square is a quadrilateral. Another says a quadrilateral is not always a square. Who is right? How could you prove it?

Teacher Notes

Listen for pupils separating has four sides (always true of a square and a quadrilateral) from has four equal sides and square corners (only the square). Revoice: "every square is a quadrilateral because it has four sides — but a quadrilateral only needs four sides, so it does not have to be a square." If pupils struggle, point back to the long thin quadrilateral built in the challenge: four sides, clearly not a square.

+10 XP

7 - What's Next ~4 mins

What we learned today

Any shape with three straight sides is a triangle, no matter how it is turned or stretched.
Any shape with four straight sides is a quadrilateral.
The trick is always to count the straight sides before naming a shape.

Coming up

Next we look at shapes with even more sides — pentagons, hexagons and octagons — and learn how their names tell you how many sides they have.

Teacher Notes

Keep this brief. A quick "three sides means…?" / "four sides means…?" call-and-response is a tidy close.

+5 XP

Pupil practice

Module 7 · 2D and 3D Shapes and Angles Data & Chance

Lesson 73 · Triangles and Quadrilaterals

Download Activity Book page (PDF)

End of lesson

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Copyright Notice
This lesson is copyright of Coding Ireland 2017 - 2025. Unauthorised use, copying or distribution is not allowed.

Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~9 mins

3 - Try It Together ~10 mins

A tall thin triangle

A wide flat triangle

A stretched quadrilateral

A five-sided shape, neither one

4 - Draw Them in Your Copy ~6 mins

5 - Class Challenge ~12 mins

Hands-on Task

6 - What Did We Notice? ~5 mins

7 - What's Next ~4 mins

What we learned today

Coming up

Unlock the full learning experience

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