Take two points A and B. Pass a line through it. Meaure it. Let it be of 6 cm. Open the compass for 6 cm,keep the pointer at A and draw an arc , now keep the pointer at B and draw an arc cutting the previous arc. Let the point of intersection of these two arcs be C. Join AC and BC. Thus a new triangle is formed ABC of 6 cm each i.e. it is an equilateral triangle. Thus an equilateral triangle can be formed on any line segment.( Just measure it and construct it ).