Sum of the angles in a triangle

Before you check the box can you see how drawing a line through the top of the triangle parallel to the base will help to show that the sum of the three angles in a triangle is 180° ?
Directions: 1. Move the vertices of ∆ABC around. 2. Notice that the angles change when you move the vertices. 3. Pull the red slider to watch the animation. 4. What can you learn form this animation?