You can illustrate the fact that the sum of the interior angles of a triangle is 180° by folding the triangle.

- Cut out a triangle of any shape. Orient the triangle so the longest side is the base.
- Fold the base onto itself so that the crease goes through the topmost vertex of the triangle. This crease will be the altitude of the triangle.
- Fold the vertex down so it touches the base at the altitude. (This crease is a midsegment of the triangle).
- Fold the other two vertices so that all three meet at the same point.
- See that all angles form a linear angle and sum to 180°.

- A simpler version of this method involves tearing off the corners and putting them together so the vertices meet.

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