Proof of the angle sum of a triangle.

Topic:
Angles
A proof that the angles in a triangle add to 180 degrees.
Which lines are parallel? Which angles are the same as the red angle in the triangle? How do you know? Which angles are the same as the blue angle in the triangle? How do you know? What must a blue, a red and a green angle add up to? How do you know? Move the vertices to change the shape of the triangle. Do your answers to any of the questions change?