Sum of the Angles in a Triangle

How can drawing a line through the top vertex of the triangle that is parallel to the base help to show that the sum of the three angles in a triangle is 180°?
Directions: 1. Move the vertices of ∆ABC around by dragging them. 2. Notice that the angles change when you move the vertices. 3. Click to place a check in the checkbox. 4. Pull the red slider to watch the animation. 5. Drag the vertices of ∆ABC again. What do you observe? What do you know about the sum of the 3 angles at the top of the triangle? What must the sum of the 3 angles of a triangle equal? Click Reset to start over.