a and b are parallel lines. They are cut by transversal c. Drag the blue points on c to move the transversal.
Note: Do not allow the blue points to fall inside the parallel lines or the angles will disappear!

Questions to Discuss:
Which angles are always congruent? Note: Congruent means same shape (same angle measure) and same size.
Which angles are vertical angles?
Which angles are adjacent angles?
Which angles are always supplementary? Think: What does supplementary mean?
Move c to the right. How does this affect the angles formed by the intersection of a and b with c? Do they get bigger or smaller? Why?
Move c to the left. How does this affect the angles formed by the intersection of a and be with c? Do they get bigger or smaller? Why?
Do the bottom four angles stay the same as the top four angles? Why or Why not?
Challenge!- Make an educated guess!
What are alternate interior angles, alternate exterior angles, and corresponding angles? Think about what each word means and make a good guess!