On the screen, we see a triangle ABC in blue color.
Observe how the shape and angles of this triangle ABC change as we move the points A, B and C. We also see line ECD which is parallel to line AB

Questions to think about
1. What is the sum of the angles <ECA+<ACB+<BCD?
2. What is the value of angle BCD? How does it compare it with angle ABC?
3. What is the relation between angles ECA and angle CAB?
4. Now, figure out how we can determine that the sum of the angles of the triangle is always 180 degrees.