http://www.youtube.com/watch?v=-Weia4Pe-9k
http://www.youtube.com/watch?v=BSduMUz2psw (optional)
http://www.regentsprep.org/Regents/math/geometry/GP9/LParallelogram.htm
Try experimenting with different parallelograms and see what happens (click the link below).
http://www.mathsisfun.com/geometry/quadrilaterals-interactive.html
Homework: Complete the attached WS 6.2 (both sides)