#### Material

2 yellow right-angled isosceles triangles with a black line painted on one of the sides enclosing the right angle.

2 green right-angled isosceles triangles equal to the yellow ones (above) with black lines on the hypotenuse of each.

2 yellow right-angled scalene triangles with black lines on the shorter of the two sides enclosing the right angle.

2 green right-angles scalene triangles, equal to the yellow scalene right-angled triangles (above) with black lines on the longer of the two sides enclosing the right angle.

2 gray right-angled scaled triangles, the same size as the yellow and green (above) with black lines on the hypotenuse.

2 yellow equilateral triangles, black lines on one side.

1 red right-angled scalene triangle with a black line on the longer of the two sides enclosing the right angle.

1 red obtuse-angled scalene triangle with a black line on the side opposite the obtuse angle.

#### Presentation

- Invite the child to the lesson.
- Remove 3 pairs of triangles.
- Place one triangle on top of the other one.
- Take a moment to examine their size.
- Take the triangle off and place it back next to the other one.
- With drama, show the child the black lines.
- Glide two fingers on the black lines of each triangle.
- Dramatically and slowly bring the pair together.
- Slide the set to the top of the rug.
- Get a contrasting triangle and repeat the process with its corresponding triangle.
- Slide it next to other set.
- Continue this process until all triangles are on the rug in a few rows.
- Bring out the red triangles last.
- Randomize the triangles.
- Invite the child to explore.

#### Exercises

Child works independently.

#### Language

None.

#### Control of Error

The black lines on each triangle.

#### Direct Purpose

To show the child that joining two triangles forms a single straight lined figure.

#### Indirect Purpose

Preparation for understanding equivalence.

To see how all plain geometric figures constructed of straight lines are composed of triangles.

#### Age

4

#### Notes

Use three contrasting sets of triangles (size and color).

The child may also use this material without connecting the black lines, i.e. the child may put together the various triangles however he likes, exploring them, making patterns, shapes, etc.