Parallelogram In Any Quadrilateral
When we connect the midpoints (the point exactly half-way along a line) of each side of the quadrilateral, one after the other, we create a new shape that has opposite sides parallel, even though the containing quadrilateral might not.
Play with it here:
To see why, click on the "Diags" button and drag the points around, while thinking "midpoints".
Also try different shape types, such as square and kite, and see what happens.