Steinhaus hinged seventh
Triangle formed by joining vertices by point at one third length on opposite side
When the angle is zero you have the original triangle. When the angle is 180˚, six triangles congruent to the pink triangle are formed.
This activity is based on the proof by Steinhaus that the area of the pink triangle is one seventh the area of the original triangle.
Reference
Steinhaus, Hugo. Mathematical snapshots. Oxford University Press, 1983.