1. gray, equilateral triangle
2. green, scalene right-angled triangles
3. yellow, isosceles obtuse-angled triangles
4. red, equilateral triangles
2. Take all triangles out and put the box away.
3. Tell the child that "I am going to build the triangle like this one", and put the gray equilateral triangle on the left bottom side.
4. Take one green triangle and put it on the bottom of the rug.
5. Looking for the same color shape triangle and slowly sliding it into the previous one by matching the black line together.
6. Take the yellow triangles and matching them as an equilateral triangle by set the black lines upward.
7. Take the red triangles and matching them as an equilateral triangle by set the black lines on the middle.
8. Put all triangles back to the box and let the child do it.