Investigate parallelograms
Explore this interactive page. The sliders allow you to change the shape of the parallelogram. Watch the measurements as you change its shape. 
Some facts are always true about every parallelogram.
As you change the shape of the parallelogram what is always true about:

the opposite sides?

the opposite angles?

the diagonals?
Do you notice any other facts that are always true about every parallelogram?
Open GeoGebra and follow these steps (DOCX 809kB) to create a new applet. 
Explore your applet:

How does the applet help you to see that the opposite sides are equal?

What does the applet demonstrate about the opposite angles?

What can you add to your applet so that it demonstrates that the diagonals have been bisected?